Light emitting device and display device including the same

ABSTRACT

A light emitting device includes: a first electrode and a second electrode with a surface facing the first electrode; an emission layer disposed between the first electrode and the second electrode and including a quantum dot (e.g., a plurality of quantum dots); and an electron auxiliary layer disposed between the emission layer and the second electrode. The electron auxiliary layer includes a first layer including a first metal oxide, and a second layer disposed on the first layer and including a second metal oxide. A roughness of an interface between the second layer and the second electrode is less than about 10 nm as determined by an electron microscopy analysis. An absolute value of a difference between a conduction band edge energy level of the second layer and a work function of the second electrode may be less than or equal to about 0.5 eV, and a conduction band edge energy level of the first layer may be less than the conduction band edge energy level of the second layer.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Korean Patent Application No.10-2019-0081628 filed in the Korean Intellectual Property Office on Jul.5, 2019, and all the benefits accruing therefrom under 35 U.S.C. § 119,the entire content of which is incorporated herein by reference.

BACKGROUND 1. Field

A light emitting device and a display device are disclosed.

2. Description of the Related Art

Unlike bulk materials, intrinsic physical characteristics (e.g., bandgapenergy or melting point.) of nanoparticles may be varied by changing thesize of the nanoparticle. A quantum dot may exhibit electroluminescentand photoluminescent properties. For example, a semiconductornanocrystal particle, also known as a quantum dot, if placed in anexcited energy state, e.g., with irradiation from a light source or withelectrical energy, e.g., an applied electric current, may emit light ina wavelength corresponding to the size of the quantum dot.

Accordingly, the quantum dot may be used as a light emitting elementthat can emit light of a particular wavelength, and is of presentinterest.

SUMMARY

Research on using a quantum dot as a light emitting element in a lightemitting device is ongoing and of great interest. Described herein isdeveloping a quantum dot and design improvements to a light emittingdevice that use quantum dots having improved performance.

An embodiment provides a light emitting device having improvedperformance.

An embodiment provides an electronic device including the light emittingdevice.

According to an embodiment, a light emitting device includes a firstelectrode and a second electrode with a surface facing the firstelectrode;

an emission layer disposed between the first electrode and the secondelectrode and including a quantum dot (e.g., a plurality of quantumdots); and

and an electron auxiliary layer disposed between the emission layer andthe second electrode,

wherein the electron auxiliary layer includes a first layer proximate tothe emission layer and including a first metal oxide, and a second layerdisposed on the first layer and proximate to the second electrode, thesecond layer including a second metal oxide,

wherein a roughness of an interface between the second layer (e.g., anopposite surface of the second layer that is disposed directly on thesurface of the second electrode) and the surface of the second electrodeis less than about 10 nanometers (nm) as determined by an electronmicroscopy analysis.

In an embodiment, an absolute value of a difference between a conductionband edge energy level of the second layer and a work function of thesecond electrode may be less than or equal to about 0.5 electron volts(eV).

In an embodiment, a conduction band edge energy level of the first layermay be less than the conduction band edge energy level of the secondlayer.

In an embodiment, the conduction band edge energy level of the secondlayer may greater than the work function of the second electrode.

In an embodiment, an energy bandgap of the second metal oxide or thesecond layer may be less than or equal to about 3.5 eV, less than orequal to about 3.4 eV, less than or equal to about 3.35 eV, or less thanor equal to about 3.3 eV.

In an embodiment, an energy bandgap of the second metal oxide or thesecond layer may be greater than or equal to about 3.0 eV, greater thanor equal to about 3.1 eV, or greater than or equal to about 3.2 eV.

An absolute value of a difference between the conduction band edgeenergy level of the second layer and the work function of the secondelectrode is less than or equal to about 0.3 eV.

An absolute value of a difference between the conduction band edgeenergy level of the second layer and the work function of the secondelectrode is less than or equal to about 0.1 eV.

The emission layer may not include cadmium, lead, mercury, or acombination thereof. The emission layer may not include cadmium.

An amount of carbon in the first layer may be greater than an amount ofcarbon in the second layer, for example, as determined by X-rayphotoelectron spectroscopy.

An amount of carbon in the first layer may be greater than about 6 molepercent (mol %) based on a total mole amount of elements included in thefirst layer.

An amount of carbon in the first layer may be greater than or equal toabout mol % based on a total mole amount of elements included in thefirst layer.

An amount of carbon in the first layer may be less than or equal toabout 60 mol % based on a total mole amount of elements included in thefirst layer.

The first metal oxide may have a composition different from that of thesecond metal oxide.

The first metal oxide may include zinc, magnesium, calcium, zirconium,yttrium, titanium, tin, tungsten, niobium, cerium, strontium, barium,indium, silicon, nickel, copper, cobalt, molybdenum, vanadium, gallium,manganese, iron, aluminum, or a combination thereof.

The first metal oxide may include a zinc oxide, a zinc magnesium oxide,a tin oxide, a titanium oxide, or a combination thereof.

The first metal oxide may include TiO₂, ZnO, SnO₂, WO₃, In₂O₃, Nb₂O₅,Fe₂O₃, CeO₂, SrTiO₃, Zn₂SnO₄, BaSnO₃, Al₂O₃, ZrO₂, SiO₂, or acombination thereof.

The first metal oxide may be represented by Chemical Formula 1:

Zn_(1-x)M_(x)O  Chemical Formula 1

In Chemical Formula 1, M is Mg, Ca, Zr, W, Li, Ti, Y, Al, or acombination thereof, and 0≤x<0.5.

The first layer may include a nanoparticle (or a plurality ofnanoparticles) of the first metal oxide.

An average particle size of the plurality of nanoparticles (of the firstmetal oxide) may be greater than or equal to about 1 nanometer (nm).

An average particle size of the plurality of nanoparticles may be lessthan or equal to about 10 nanometers.

In one embodiment, a surface of the second layer may be disposeddirectly on a surface of the first layer.

An amount of carbon in the second layer may be less than or equal toabout 12 mol % based on a total mole amount of elements included in thesecond layer. An amount of carbon in the second layer may be less thanor equal to about 10 mol % based on a total mole amount of elementsincluded in the second layer. An amount of carbon in the second layermay be less than or equal to about 8 mol % based on a total mole amountof elements included in the second layer. An amount of carbon in thesecond layer may be less than or equal to about 6 mol % based on a totalmole amount of elements included in the second layer.

An amount of carbon in the second layer may be greater than or equal to0 mol % based on a total mole amount of elements included in the secondlayer.

An amount of carbon in the second layer may be greater than or equal to0.1 mol % based on a total mole amount of elements included in thesecond layer.

A surface roughness of an interface between a surface of the secondlayer and a surface of the first layer may be less than or equal toabout 12 nm as determined by an electron microscopy analysis.

A surface roughness of an interface between a surface of the secondlayer and a surface of the first layer may be less than or equal toabout 10 nm as determined by an electron microscopy analysis.

A surface roughness of an interface between a surface of the secondlayer and a surface of the first layer may be less than or equal toabout 9 nm as determined by an electron microscopy analysis.

A surface roughness of an interface between a surface of the secondlayer and a surface of the first layer may be less than or equal toabout 8 nm as determined by an electron microscopy analysis.

The second metal oxide may include zinc, magnesium, calcium, zirconium,yttrium, titanium, tin, tungsten, niobium, cerium, strontium, barium,indium, silicon, nickel, copper, cobalt, molybdenum, vanadium, gallium,manganese, iron, aluminum, or a combination thereof.

The second metal oxide may include TiO₂, ZnO, SnO₂, WO₃, In₂O₃, Nb₂O₅,Fe₂O₃, CeO₂, SrTiO₃, Zn₂SnO₄, BaSnO₃, Al₂O₃, ZrO₂, SiO₂, or acombination thereof.

The second metal oxide may include a zinc oxide, a zinc magnesium oxide,a tin oxide, a titanium oxide, or a combination thereof.

The second metal oxide may be represented by Chemical Formula 2:

Zn_(1-x)M_(x)O  Chemical Formula 2

In Chemical Formula 2, M is Mg, Ca, Zr, W, Li, Ti, Y, Al, or acombination thereof, and 0≤x<0.5.

A roughness of an interface between an opposite surface of the secondlayer and the surface of the second electrode is less than about 5 nm asdetermined by an electron microscopy analysis.

A roughness of an interface between an opposite surface of the secondlayer and the surface of the second electrode is less than about 3 nm asdetermined by an electron microscopy analysis.

A thickness of the second layer may be greater than or equal to about 1nm. A thickness of the second layer may be less than or equal to about30 nm.

A difference between a conduction band edge energy level of the firstlayer and a conduction band edge energy level of the second layer may begreater than or equal to about 0.05 eV.

A difference between a conduction band edge energy level of the firstlayer and a conduction band edge energy level of the second layer may begreater than or equal to about 0.1 eV.

A difference between a conduction band edge energy level of the firstlayer and a conduction band edge energy level of the second layer may beless than or equal to about 1 eV.

In one embodiment, the light emitting device may further include abuffer layer disposed on an opposite surface of the second electrode,and optionally on an opposite surface of the second layer, the bufferlayer including an organic metal compound, a metal fluoride (e.g.,lithium fluoride), or a combination thereof.

The metal of the organic metal compound or the metal fluoride mayinclude lithium, aluminum, or a combination thereof.

The metal fluoride may include lithium fluoride.

The organic moiety of the organic metal compound may include an aromaticcyclic moiety, a heteroaromatic cyclic moiety, each of which optionallymay include fluorine, or a combination thereof.

The organic metal compound may include 8-hydroxyquinolatolithium (Liq),tris(8-hydroxyquinolinato)aluminium (Alq3), or a combination thereof.

The buffer layer may have an electrical conductivity of less than orequal to about 3×10⁻⁴ Siemens per centimeter (S/cm).

A thickness of the buffer layer may be greater than or equal to about 3nm, greater than or equal to about 5 nm, greater than or equal to about7 nm.

The light emitting device may further include an organic polymer layerdisposed on (e.g., an opposite surface of) the second electrode.

The organic polymer layer may not contact with a major surface of thesecond layer.

The organic polymer layer may include a polymerization product of amonomer combination (e.g., a monomer composition) including a compoundhaving at least one carbon-carbon double bond.

The monomer combination may further include a thiol compound.

The organic polymer layer may not include (meth)acrylic acid, benzoicacid, 3-butenoic acid, crotonic acid, butyric acid, isobutyric acid,acetic acid, propionic acid, a polymer thereof, or a combination thereof

The organic polymer layer may cover (e.g., encapsulate) the firstelectrode, the second electrode, the emission layer, and the electronauxiliary layer.

The light emitting device may emit blue light, and a T50 of the lightemitting device may be greater than or equal to about 10 hours.

According to an embodiment, a display device includes the aforementionedlight emitting device.

In the device (e.g., electroluminescent device), an electron injectionfrom a cathode may be efficiently carried out and a movement(extraction) of holes toward the anode may be effectively prohibited,and an electron transport rate may be controlled appropriately, wherebya carrier recombination at an interface and a bulk type non-emissiverecombination may be reduced. Accordingly, the device of the embodimentsmay exhibit increased luminous efficiency and improved lifetime.

The layered structure included in the device of the embodiments may beused in various semiconductor devices such as a light emitting diodedevice, a sensor, a laser device, a solar cell device, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

The above and other advantages and features of this disclosure willbecome more apparent by describing in further detail exemplaryembodiments thereof with reference to the accompanying drawings, inwhich:

FIG. 1 is a schematic cross-sectional view of a non-limiting embodimentof a light emitting device;

FIG. 2 is a schematic cross-sectional view of a non-limiting embodimentof a light emitting device

FIG. 3 is a schematic representation of an energy band diagram of anelectrode, a first electron auxiliary layer, a second electron auxiliarylayer, and an emission layer and a carrier flow a, b, c, and d, that mayoccur in a device of an embodiment;

FIG. 4A is a schematic cross-sectional illustration of a device preparedin the Examples and Comparative Examples.

FIG. 4B is another schematic cross-sectional illustration of a deviceprepared in the Examples and Comparative Examples.

FIG. 5A is a Transmission Electron Microscopy (TEM) image of across-section of a device of Example 1.

FIG. 5B is a view showing results of TEM-EDX analysis for thecross-section of the device of Example 1.

FIG. 6 is a TEM image of a cross-section of a layered structure of afirst layer and a second layer formed on a Si substrate by the samemethod as Comparative Example 1.

FIG. 7 is a graph plotting the electroluminescent properties (LuminanceCd/m² Vs EQE %) for the device of Example 2 and Comparative Example 2.

FIG. 8 is a graph plotting the electroluminescent properties (LuminanceCd/m² Vs EQE %) for the device of Example 3 and Comparative Example 3.

FIG. 9A is a picture of emission of the device without a buffer layer inExperimental Example 1 prior to and after an oven aging at a temperatureof 70° C. for 10 days.

FIG. 9B is a picture of emission of the device with a buffer layer inExperimental Example 2 prior to and after an oven aging at a temperatureof 70° C. for 10 days.

FIG. 10 is a view showing electroluminescent properties for the devicewith a buffer layer in Experimental Example 3 prior to and after an ovenaging at a temperature of 70° C. for 5 days.

DETAILED DESCRIPTION

Advantages and characteristics of this disclosure, and a method forachieving the same, will become evident referring to the followingexample embodiments together with the accompanying drawings so that aperson skilled in the art would understand the same. This disclosuremay, however, be embodied in many different forms and is not construedas limited to the example embodiments set forth herein. Rather, theseembodiments are provided so that this disclosure will be thorough andcomplete, and will fully convey the scope of the invention to thoseskilled in the art. In the drawings, the thickness of layers, films,panels, regions, etc., are exaggerated for clarity. Like referencenumerals designate like elements throughout the specification. It willbe understood that when an element such as a layer, film, region, orsubstrate is referred to as being “on” another element, it can bedirectly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” another element, there are no intervening elements present.

It will be understood that, although the terms “first,” “second,”“third,” etc. may be used herein to describe various elements,components, regions, layers, and/or sections, these elements,components, regions, layers, and/or sections should not be limited bythese terms. These terms are only used to distinguish one element,component, region, layer, or section from another element, component,region, layer, or section. Thus, “a first element,” “component,”“region,” “layer” or “section” discussed below could be termed a secondelement, component, region, layer, or section without departing from theteachings herein.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting. As used herein, thesingular forms “a,” “an,” and “the” are intended to include the pluralforms, including “at least one,” unless the content clearly indicatesotherwise. “At least one” is not to be construed as limiting “a” or“an.” “Or” means “and/or.” As used herein, the term “and/or” includesany and all combinations of one or more of the associated listed items.

It will be further understood that the terms “comprises” and/or“comprising,” or “includes” and/or “including” when used in thisspecification, specify the presence of stated features, regions,integers, steps, operations, elements, and/or components, but do notpreclude the presence or addition of one or more other features,regions, integers, steps, operations, elements, components, and/orgroups thereof.

“About” or “approximately” as used herein is inclusive of the statedvalue and means within an acceptable range of deviation for theparticular value as determined by one of ordinary skill in the art,considering the measurement in question and the error associated withmeasurement of the particular quantity (i.e., the limitations of themeasurement system). For example, “about” can mean within one or morestandard deviations, or within ±10% or 5% of the stated value.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this disclosure belongs. It willbe further understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art and thepresent disclosure, and will not be interpreted in an idealized oroverly formal sense unless expressly so defined herein.

Exemplary embodiments are described herein with reference to crosssection illustrations that are schematic illustrations of idealizedembodiments. As such, variations from the shapes of the illustrations asa result, for example, of manufacturing techniques and/or tolerances,are to be expected. Thus, embodiments described herein should not beconstrued as limited to the particular shapes of regions as illustratedherein but are to include deviations in shapes that result, for example,from manufacturing. For example, a region illustrated or described asflat may, typically, have rough and/or nonlinear features. Moreover,sharp angles that are illustrated may be rounded. Thus, the regionsillustrated in the figures are schematic in nature and their shapes arenot intended to illustrate the precise shape of a region and are notintended to limit the scope of the present claims.

As used herein, the expression that “not including cadmium (or otherheavy metal)” or “substantially free of cadmium (or other heavy metal)”include the case where a concentration of the cadmium (or other heavymetal) may be less than or equal to about 100 ppm, less than or equal toabout 50 ppm, less than or equal to about 10 ppm, or almost zero. In anembodiment, substantially no amount of the cadmium (or other heavymetal) is present or, if present, an amount of the cadmium (or otherheavy metal) is less than or equal to a detection limit or as animpurity level of a given analysis tool (e.g., an inductively coupledplasma atomic emission spectroscopy).

As used herein, the term a work function, a conduction band (edge)energy level, or a highest occupied molecular orbital (“HOMO”) or alowest unoccupied molecular orbital (LUMO) energy level is expressed asan absolute value from a vacuum.

In addition, if a work function, a conduction band energy level, a HOMOenergy level or a LUMO energy level is said to be ‘deep,’ ‘high’ or‘large,’ the work function, the conduction band energy level, the HOMOenergy level, or the LUMO energy level has a large absolute valuerelative to ‘0 eV,’ i.e., the energy level of a vacuum. In contrast, ifthe work function, the conduction band energy level, the HOMO energylevel, or the LUMO energy level is said to be ‘shallow,’ ‘low,’ or‘small,’ the work function, the conduction band energy level, the HOMOenergy level, or the LUMO energy level has a small absolute value from‘0 eV,’ i.e., the energy level of a vacuum.

As used herein, the word “Group” refers to a group in the periodic tableof the elements.

As used herein, the term “Group II” refers to Group IIA and Group IIB ofthe Periodic Table of the elements, and examples of Group II metals mayinclude Cd, Zn, Hg, and Mg, but are not limited thereto.

As used herein, the term “Group III” refers to Group IIIA and Group IIIBof the Periodic Table of the elements, and examples of Group III metalsmay include Al, In, Ga, and TI, but are not limited thereto.

As used herein, the term “Group IV” may refer to Group IVA and Group IVBof the Periodic Table of the elements, and examples of a Group IV metalmay include Si, Ge, and Sn, but are not limited thereto.

As used herein, the term “metal” refers to metallic or metalloidelements as defined in the Periodic Table of Elements selected fromGroups 1 to 17 of the Periodic Table of the elements, including thelanthanide elements and the actinide elements, and includes a semi-metalsuch as Si.

As used herein, the term “Group I” may refer to Group IA and Group IB ofthe Periodic Table of the elements, and examples may include Li, Na, K,Rb, and Cs, but are not limited thereto.

As used herein, the term “Group V” may refer to Group VA of the PeriodicTable of the elements, and examples may include nitrogen, phosphorus,arsenic, antimony, and bismuth, but are not limited thereto.

As used herein, the term “Group VI” may refer to Group VIA of thePeriodic Table of the elements, and examples may include sulfur,selenium, and tellurium, but are not limited thereto.

As used herein, unless a definition is otherwise provided, “substituted”refers to the replacement of hydrogen of a compound, a group, or amoiety by at least one (e.g., 1, 2, 3, or 4) substituent independentlyselected from a C1 to C30 alkyl group, a C2 to C30 alkenyl group, a C2to C30 alkynyl group, a C2 to C30 epoxy group, a C2 to C30 alkyl estergroup, a C3 to C30 alkenyl ester group (e.g., acrylate group,methacrylate group), a C6 to C30 aryl group, a C7 to C30 alkylarylgroup, a C1 to C30 alkoxy group, a C1 to C30 heteroalkyl group, a C3 toC30 heteroalkylaryl group, a C3 to C30 cycloalkyl group, a C3 to C15cycloalkenyl group, a C6 to C30 cycloalkynyl group, a C2 to C30heterocycloalkyl group, a halogen (—F, —Cl, —Br, or —I), a hydroxy group(—OH), a nitro group (—NO₂), a cyano group (—CN), an amino group (—NRR′wherein R and R′ are independently hydrogen or a C1 to C6 alkyl group),an azido group (—N₃), an amidino group (—C(═NH)NH₂), a hydrazino group(—NHNH₂), a hydrazono group (═N(NH₂)), an aldehyde group (—C(═O)H), acarbamoyl group (—C(O)NH₂), a thiol group (—SH), an ester group(—C(═O)OR or RC(═O)O—, wherein R is a C1 to C6 alkyl group or a C6 toC12 aryl group), a carboxyl group (—COOH) or a salt thereof (—C(═O)OM,wherein M is an organic or inorganic cation), a sulfonic acid group(—SO₃H) or a salt thereof (—SO₃M, wherein M is an organic or inorganiccation), a phosphoric acid group (—PO₃H₂) or a salt thereof (—PO₃MH or—PO₃M₂, wherein M is an organic or inorganic cation), and a combinationthereof.

Hereinafter, a light emitting device according to an embodiment isfurther described with reference to the drawings.

FIG. 1 is a schematic cross-sectional view of an embodiment of a lightemitting device.

Referring to FIG. 1 , a light emitting device according to an embodimentincludes a first electrode 11 and a second electrode 15 having a surfacefacing the first electrode 11, an emission layer 13 disposed between thefirst electrode 11 and the second electrode 15, and including a quantumdot (e.g. a plurality of quantum dots), and an electron auxiliary layer14 disposed between the second electrode 15 and the emission layer 13wherein the electron auxiliary layer may include a first layer 14 adisposed proximate (e.g., directly adjacent) to the emission layer andincluding a first metal oxide; and a second layer 14 b disposed on thefirst layer 14 a and proximate (e.g., directly adjacent) to the secondelectrode, and including a second metal oxide. The device may include ahole auxiliary layer 12 disposed between the first electrode 11 and theemission layer 13. The hole auxiliary later 12 may include a holetransport layer 12 b proximate to the emission layer, and a holeinjection layer 12 a proximate to the first electrode 11.

A substrate (not shown) may be disposed on a surface of the firstelectrode 11 or an opposite surface of the second electrode 15, i.e. thesurface not facing the first electrode 11. In an embodiment, thesubstrate may be disposed on a surface of the first electrode (e.g.,under or below the first electrode of FIG. 1 ). The substrate mayinclude an insulating material (e.g., insulating transparent substrate).The substrate may include glass; a polymer such as a polyester (e.g.,polyethylene terephthalate (PET), polyethylene naphthalate (PEN)), apolycarbonate, a polyacrylate, a polyimide, a poly(amide-imide), apolysiloxane (e.g. PDMS), or a combination thereof; an inorganicmaterial such as Al₂O₃, ZnO, or a combination thereof; or a combinationcomprising a least two of the foregoing, but is not limited thereto. Thesubstrate may be made of a silicon wafer. As used herein, the term“transparent” refers to having a transmittance of greater than or equalto about 85% transmittance of light having a predetermined wavelength(e.g., light emitted from a quantum dot), or for example, transmittanceof greater than or equal to about 88%, greater than or equal to about90%, greater than or equal to about 95%, greater than or equal to about97%, or greater than or equal to about 99%, e.g., about 85% to about99.99%, or about 90% to about 99.9%. A thickness of the substrate may beappropriately selected considering a substrate material but is notparticularly limited. The transparent substrate may be flexible. Thesubstrate may be omitted.

One of the first electrode 11 or the second electrode may be an anodeand the other may be a cathode. In an embodiment, the first electrode 11may be an anode and the second electrode 15 may be a cathode.

The first electrode 11 may be made of an electrically conductivematerial, for example a metal, a conductive metal oxide, or acombination thereof, e.g., an electrically conductive material known inthe art. The first electrode 11 may include, for example, a metal or analloy thereof, the metal including nickel, platinum, vanadium, chromium,copper, zinc, and gold; a conductive metal oxide such as zinc oxide,indium oxide, tin oxide, indium tin oxide (ITO), indium zinc oxide(IZO), or fluorine doped tin oxide; or a combination of metal and ametal oxide such as ZnO and Al or SnO₂ and Sb, but is not limitedthereto. A combination comprising at least two of the foregoing may alsobe used. In an embodiment, the first electrode may include a transparentconductive metal oxide, for example, indium tin oxide. A work functionof the first electrode may be greater than a work function of the secondelectrode. Alternatively, a work function of the first electrode may beless than a work function of the second electrode.

The second electrode 15 may include a conductive material, for example ametal, a conductive metal oxide, a conductive polymer, or a combinationthereof, e.g., an electrically conductive material known in the art. Thesecond electrode 15 may include, for example, a metal or an alloythereof, such as aluminum, magnesium, calcium, sodium, potassium,titanium, indium, yttrium, lithium, gadolinium, silver, gold, platinum,tin, lead, cesium, or barium; a multi-layer structured material such asLiF/Al, Li₂O/Al, 8-hydroxyquinolinolato-lithium/aluminum (Liq/Al),LiF/Ca, or BaF₂/Ca, but is not limited thereto. A combination comprisingat least two of the foregoing may also be used. Details for theconductive metal oxide are the same as described above.

In an embodiment, a work function of the first electrode (e.g., ananode) may be greater than or equal to about 4.0 electron volts (eV),greater than or equal to about 4.1 eV, greater than or equal to about4.2 eV, greater than or equal to to about 4.3 eV, greater than or equalto about 4.4 eV, greater than or equal to about 4.5 eV, greater than orequal to about 4.6 eV, greater than or equal to about 4.7 eV, or greaterthan or equal to about 4.8 eV. A work function of the first electrode(e.g., an anode) may be less than or equal to about 5.5 eV, less than orequal to about 5.4 eV, less than or equal to about 5.3 eV, less than orequal to about 5.2 eV, less than or equal to about 5.1 eV, less than orequal to about 5.0 eV, or less than or equal to about 4.9 eV.

In an embodiment, a work function of the second electrode (e.g., ancathode) may be greater than or equal to about 3.4 eV, greater than orequal to about 3.5 eV, greater than or equal to about 3.6 eV, greaterthan or equal to about 3.7 eV, greater than or equal to about 3.8 eV,greater than or equal to about 3.9 eV, greater than or equal to about4.0 eV, greater than or equal to about 4.1 eV, greater than or equal toabout 4.2 eV, greater than or equal to about 4.3 eV, greater than orequal to about 4.4 eV, or greater than or equal to about 4.5 eV. A workfunction of the second electrode (e.g., an cathode) may be less than orequal to about 5.0 eV, less than or equal to about 4.9 eV, less than orequal to about 4.8 eV, less than or equal to about 4.7 eV, less than orequal to about 4.6 eV, less than or equal to about 4.5 eV, or less thanor equal to about 4.4 eV.

At least one of the first electrode 11 or the second electrode 15 may bea light-transmitting electrode and the light-transmitting electrode maybe for example made of a conductive oxide such as a zinc oxide, anindium oxide, a tin oxide, an indium tin oxide (ITO), an indium zincoxide (IZO), a fluorine doped tin oxide, a metal thin layer including asingle layer or a multilayer, or a combination thereof. If one of thefirst electrode 11 or the second electrode 15 is anon-light-transmitting (e.g., non-transparent) electrode, it mayinclude, for example, an opaque conductive material such as aluminum(Al), silver (Ag), gold (Au), or a combination thereof.

A thickness of the electrodes (the first electrode and/or the secondelectrode) is not particularly limited and may be appropriately selectedwith consideration of the device efficiency. For example, the thicknessof the electrodes may be greater than or equal to about 5 nanometers(nm), for example, greater than or equal to about 50 nm, greater than orequal to about 100 nm, greater than or equal to about 500 nm, or greaterthan or equal to about 1 μm. For example, the thickness of theelectrodes may be less than or equal to about 100 micrometers (μm), forexample, less than or equal to about 10 μm, less than or equal to about1 μm, less than or equal to about 900 nm, less than or equal to about500 nm, or less than or equal to about 100 nm.

The emissive layer 13 includes a quantum dot (e.g., a plurality ofquantum dots). The quantum dot (at times, also referred to herein as asemiconductor nanocrystal) may have a core-shell structure including acore including a first semiconductor nanocrystal and a shell disposed onthe core and including a second semiconductor nanocrystal different fromthe first semiconductor nanocrystal.

The quantum dot (e.g., the first semiconductor nanocrystal and/or thesecond semiconductor nanocrystal) may include a Group II-VI compound, aGroup III-V compound, a Group IV-VI compound, a Group IV element orcompound, a Group compound, a Group compound, a Group I-II-IV-VIcompound, or a combination thereof. The quantum dot (the firstsemiconductor nanocrystal, the second semiconductor nanocrystal, or acombination thereof) may not include a harmful heavy metal (e.g.,cadmium, lead, mercury, or a combination thereof). For example, theplurality of the quantum dots do not include or are substantially freeof harmful heavy metal (e.g., cadmium, lead, mercury, or a combinationthereof). For instance, the plurality of the quantum dots do not includecadmium.

The Group II-VI compound may include a binary element compound includingCdSe, CdTe, ZnS, ZnSe, ZnTe, ZnO, HgS, HgSe, HgTe, MgSe, MgS, or acombination thereof; a ternary element compound including CdSeS, CdSeTe,CdSTe, ZnSeS, ZnSeTe, ZnSTe, HgSeS, HgSeTe, HgSTe, CdZnS, CdZnSe,CdZnTe, CdHgS, CdHgSe, CdHgTe, HgZnS, HgZnSe, HgZnTe, MgZnSe, MgZnS, ora combination thereof; or a quaternary element compound includingHgZnTeS, CdZnSeS, CdZnSeTe, CdZnSTe, CdHgSeS, CdHgSeTe, CdHgSTe,HgZnSeS, HgZnSeTe, HgZnSTe, ora combination thereof. The Group II-VIcompound may further include a Group III metal. A combination comprisingat least two of the foregoing may also be used.

The Group III-V compound may include a binary element compound includingGaN, GaP, GaAs, GaSb, AlN, AlP, AlAs, AlSb, InN, InP, InAs, InSb, or acombination thereof; a ternary element compound including GaNP, GaNAs,GaNSb, GaPAs, GaPSb, AlNP, AlNAs, AlNSb, AlPAs, AlPSb, InNP, InNAs,InNSb, InPAs, InPSb, or a combination thereof; or a quaternary elementcompound including GaAlNP, GaAlNAs, GaAlNSb, GaAlPAs, GaAlPSb, GaInNP,GaInNAs, GaInNSb, GaInPAs, GaInPSb, InAlNP, InAlNAs, InAlNSb, InAlPAs,InAlPSb, or a combination thereof. The Group III-V compound may furtherinclude a Group II metal (e.g., InZnP).

The Group IV-VI compound may include a binary element compound includingSnS, SnSe, SnTe, PbS, PbSe, PbTe, or a combination thereof; a ternaryelement compound including SnSeS, SnSeTe, SnSTe, PbSeS, PbSeTe, PbSTe,SnPbS, SnPbSe, SnPbTe, or a combination thereof; or a quaternary elementcompound including SnPbSSe, SnPbSeTe, SnPbSTe, ora combination thereof.

Examples of the Group compound may include CuInSe₂, CuInS₂, CuInGaSe, orCuInGaS, but are not limited thereto. Examples of the Group VI compoundmay include CuZnSnSe or CuZnSnS, but are not limited thereto. The GroupIV element or compound may include a single substance selected from Si,Ge, or a combination thereof; or a binary element compound includingSiC, SiGe, or a combination thereof. A combination comprising at leasttwo of the foregoing may also be used.

In an embodiment, the first semiconductor nanocrystal may include ametal including indium, zinc, or a combination thereof; and a non-metalincluding phosphorous, selenium, tellurium, sulfur, or a combinationthereof. In an embodiment, the second semiconductor nanocrystal mayinclude a metal including indium, zinc, or a combination thereof; and anon-metal including phosphorous, selenium, tellurium, sulfur, or acombination thereof.

In an embodiment, the first semiconductor nanocrystal may include InP,InZnP, ZnSe, ZnSeS, ZnSeTe, or a combination thereof. In an embodiment,the second semiconductor nanocrystal may include ZnSe, ZnSeS, ZnS,ZnTeSe, or a combination thereof. In an embodiment, the quantum dots mayemit blue light or green light and the quantum dots may include a coreof ZnSeTe, ZnSe, or a combination thereof and a shell including Zn, Se,and S (e.g., ZnSeS). In the shell, the amount (or the concentration) ofthe sulfur may vary (e.g., increase or decrease) in a direction ofradius (e.g. from the core toward a surface of the surface of thequantum dot). In an embodiment, the shell may include zinc, sulfur, andoptionally selenium (e.g., at an outermost layer thereof). In anembodiment, the quantum dots may emit red light or green light, and thecore may include InP, InZnP, or a combination thereof, and the shell mayinclude a Group II metal including zinc and a non-metal including atleast one of sulfur and selenium.

In an embodiment, the quantum dot may have a core-shell structure and aninterface between the core and the shell may include or may not includean alloyed layer. The alloyed layer may be a homogeneous alloy or agradient alloy. In the gradient alloy, a concentration of an element ofthe shell may vary with a radial direction (e.g., decrease or increasetoward the core.

In an embodiment, the shell may have a composition varying in a radialdirection. In an embodiment, the shell may be a multi-layered shellhaving at least two (shell) layers. In the multi-layered shell, twoadjacent layers may have a composition different with each other. In themulti-layered shell, at least one layer may include independently analloyed semiconductor nanocrystal. In the multi-layered shell, at leastone layer may a composition varying in a radial direction.

In the core-shell structured quantum dot, the shell material and thecore material may have different energy bandgaps from each other. In anembodiment, the energy bandgap of the shell material may be greater thanthat of the core material but is not limited thereto.

According to another embodiment, the energy bandgap of the shellmaterial may be less than that of the core material. If the quantum dotdoes include a multi-layered shell, in the multi-layered shell, theenergy bandgap of the outer layer may be greater than the energy bandgapof the inner layer (i.e., the layer nearer to the core). In themulti-layered shell, the energy bandgap of the outer layer may be lessthan the energy bandgap of the inner layer. In the multi-layered shell,an energy bandgap of the semiconductor nanocrystal of each layer may beselected appropriately in order for the quantum dot to efficientlyexhibit a quantum confinement effect.

The semiconductor nanocrystal shell may include a first shell layerincluding zinc and selenium and a second shell layer disposed on thefirst shell layer and including zinc and sulfur. The first shell layermay be disposed directly on the core. The first shell layer may includeZnSe, ZnSeS, or a combination thereof. The first shell layer may notinclude sulfur.

The second shell layer may include ZnS. The second shell layer may notinclude selenium. The second shell layer may be disposed directly on thefirst layer. The second shell layer may be an outermost layer of thequantum dot.

The quantum dot may have a quantum efficiency of greater than or equalto about 10%, 20%, 30%, 40%, 50%, 60%, or 70%, greater than or equal toabout 80%, or greater than or equal to about 90%. The quantum dot mayexhibit a maximum photoluminescence peak with a full width at halfmaximum of less than or equal to about 50 nm, less than or equal toabout 45 nm, less than or equal to about 35 nm, or less than or equal toabout 30 nm.

In an embodiment, “quantum yield (or quantum efficiency)” is a ratio ofphotons emitted to photons absorbed, e.g., by a nanostructure orpopulation of nanostructures. In an embodiment, the quantum efficiencymay be determined by any method. For example, there may be two methodsfor measuring the fluorescence quantum yield or efficiency: the absolutemethod and the relative method. The absolute method directly obtains thequantum yield by detecting all sample fluorescence through the use of anintegrating sphere. The relative method compares the fluorescenceintensity of a standard sample with the fluorescence intensity of anunknown sample to calculate the quantum yield of the unknown sample. TheQY may be readily determined by using commercially available equipment.

The quantum dot may include an organic ligand (e.g., a ligand having ahydrophobic moiety or a hydrophilic moiety) and optionally a halogenmoiety for example, on a surface thereof. The organic ligand andoptionally the halogen moiety may be attached (e.g., bound) to a surfaceof the quantum dot.

The organic ligand may include RCOOH, RNH₂, R₂NH, R₃N, RSH, R₃PO, R₃P,ROH, RCOOR, RPO(OH)₂, RHPOOH, R₂POOH, or a combination thereof, wherein,R is independently a C3 to C40 substituted or unsubstituted aliphatichydrocarbon group, such as a C3 to C40 (e.g., C5 or greater and C24 orless) substituted or unsubstituted alkyl, a C3 to C40 substituted orunsubstituted alkenyl, a C6 to C40 (e.g., C6 or greater and C20 or less)substituted or unsubstituted aromatic hydrocarbon group, such as asubstituted or unsubstituted C6 to C40 aryl group, or a combinationthereof.

Examples of the organic ligand may include a thiol compound such asmethane thiol, ethane thiol, propane thiol, butane thiol, pentane thiol,hexane thiol, octane thiol, dodecane thiol, hexadecane thiol, octadecanethiol, or benzyl thiol; an amine compound such as methane amine, ethaneamine, propane amine, butane amine, pentyl amine, hexyl amine, octylamine, nonyl amine, decyl amine, dodecyl amine, hexadecyl amine,octadecyl amine, dimethyl amine, diethyl amine, dipropyl amine,tributylamine, or trioctylamine; a carboxylic acid compound such asmethanoic acid, ethanoic acid, propanoic acid, butanoic acid, pentanoicacid, hexanoic acid, heptanoic acid, octanoic acid, dodecanoic acid,hexadecanoic acid, octadecanoic acid, oleic acid, or benzoic acid; aphosphine compound such as methyl phosphine, ethyl phosphine, propylphosphine, butyl phosphine, pentyl phosphine, octylphosphine, dioctylphosphine, tributylphosphine, or trioctylphosphine; an oxide of aphosphine compound such methyl phosphine oxide, ethyl phosphine oxide,propyl phosphine oxide, butyl phosphine oxide, pentyl phosphine oxide,tributylphosphine oxide, octyl phosphine oxide, dioctyl phosphine oxide,or trioctyl phosphine oxide; a diphenyl phosphine compound, a triphenylphosphine compound, or an oxide compound thereof; C5 to C20 alkylphosphinic acid such as hexylphosphinic acid, octylphosphinic acid,dodecanephosphinic acid, tetradecanephosphinic acid,hexadecanephosphinic acid, octadecanephosphinic acid; a C5 to C20 alkylphosphonic acid such as hexyl phosphonic acid, octyl phosphonic acid,dodecane phosphonic acid, tetradecane phosphonic acid, hexadecanephosphonic acid, octadecane phosphonic acid; but are not limitedthereto. A combination comprising at least two of the foregoing may alsobe used. The quantum dot may include a hydrophobic organic ligand eitheralone or in a combination comprising two or more hydrophobic ligands.The hydrophobic organic ligand may not include a photopolymerizablemoiety (e.g., acrylate group, methacrylate group, etc.).

The halogen moiety may include chlorine, bromine, iodine, or acombination thereof. In an embodiment, the halogen moiety may includechlorine.

In an embodiment, the quantum dot may include a first organic ligand andhalogen on a surface thereof. The first organic ligand may include a C6to C40 aliphatic carboxylic acid compound (e.g., myristic acid, oleicacid, stearic acid, or the like). The carboxylic acid compound mayinclude a compound represented by RCOOH (wherein R is a C12 or greateralkyl group or a C12 or greater alkenyl group).

In an embodiment, the quantum dot(s) may include halogen, and an amountof the halogen may be greater than or equal to about 1 microgram permilligram of quantum dots (ug/mg QD), for example, greater than or equalto about 2 ug/mg QD, greater than or equal to about 3 ug/mg QD, greaterthan or equal to about 4 ug/mg QD, greater than or equal to about 5ug/mg QD, greater than or equal to about 6 ug/mg QD, or greater than orequal to about 7 ug/mg QD and less than about 30 ug/mg QD, less than orequal to about 25 ug/mg QD, less than or equal to about 20 ug/mg QD,less than or equal to about 12.4 ug/mg QD, less than or equal to about12.3 ug/mg QD, less than or equal to about 12.2 ug/mg QD, less than orequal to about 12.1 ug/mg QD, less than or equal to about 12 ug/mg QD,less than or equal to about 11.9 ug/mg QD, or less than or equal toabout 11.8 ug/mg QD. The mole ratio of the halogen (e.g., chlorine)relative to the organic ligand (e.g., fatty acid such as oleic acid) maybe less than about 2.2, for example, less than or equal to about 2, lessthan or equal to about 1.9, less than or equal to about 1.8, less thanor equal to about 1.7, or less than or equal to about 1.6. The moleratio of the halogen relative to the organic ligand may be greater thanor equal to about 0.5, for example, greater than or equal to about 0.6,greater than or equal to about 0.7, greater than or equal to about 0.8,or greater than or equal to about 0.9. The quantum dot including thehalogen moiety may be prepared by contacting the quantum dots with ahalogen (e.g. an organic or aqueous solution including the halogen) inan organic dispersion.

The absorption/photoluminescence wavelengths of the quantum dot may bemodified by selecting a composition and a size of the quantum dot. Amaximum photoluminescence peak wavelength of the quantum dot may bewithin an ultraviolet (UV) to infrared wavelength or may be a wavelengthgreater than the above wavelength range.

In an embodiment, the maximum photoluminescence peak wavelength of thequantum dot may be greater than or equal to about 300 nm, greater thanor equal to about 430 nm, greater than or equal to about 440 nm, greaterthan or equal to about 500 nm, greater than or equal to about 510 nm,greater than or equal to about 520 nm, greater than or equal to about530 nm, greater than or equal to about 540 nm, greater than or equal toabout 550 nm, greater than or equal to about 560 nm, greater than orequal to about 570 nm, greater than or equal to about 580 nm, greaterthan or equal to about 590 nm, greater than or equal to about 600 nm, orgreater than or equal to about 610 nm.

In an embodiment, the maximum photoluminescence peak wavelength of thequantum dot may be less than or equal to about 800 nm, less than orequal to about 650 nm, less than or equal to about 640 nm, less than orequal to about 630 nm, less than or equal to about 620 nm, less than orequal to about 610 nm, less than or equal to about 600 nm, less than orequal to about 590 nm, less than or equal to about 580 nm, less than orequal to about 570 nm, less than or equal to about 560 nm, less than orequal to about 550 nm, less than or equal to about 540 nm, less than orequal to about 480 nm, less than or equal to about 475 nm, or less thanor equal to about 470 nm.

In an embodiment, the maximum photoluminescence peak wavelength of thequantum dot may be from about 500 nm to about 650 nm.

In an embodiment, the quantum dot may emit green light and the maximumphotoluminescence peak wavelength of the quantum dot may be from about500 nm to about 550 nm. In an embodiment, the quantum dot may emit redlight and the maximum photoluminescence peak wavelength of the quantumdot may be from about 600 nm to about 650 nm. In an embodiment, thequantum dot may emit blue light and the maximum photoluminescence peakwavelength of the quantum dot may be from about 440 nm to about 480 nm.

The quantum dot may have quantum efficiency of greater than or equal toabout 10%, for example, greater than or equal to about 30%, greater thanor equal to about 50%, greater than or equal to about 60%, greater thanor equal to about 70%, greater than or equal to about 90%, or even about100%.

The quantum dot may have a relatively narrow emission spectrum. Thequantum dot may have, for example, a full width at half maximum (FWHM)of a photoluminescence wavelength spectrum of less than or equal toabout 50 nm, for example, less than or equal to about 45 nm, less thanor equal to about 40 nm, or less than or equal to about 30 nm.

The quantum dot(s) may have a particle size (or an average particlesize) of greater than or equal to about 1 nm and less than or equal toabout 100 nm. The particle size may refer to a diameter or an equivalentdiameter which is calculated under the assumption it has a sphericalshape based upon a 2D image obtained by (e.g., transmission) electronmicroscope analysis. The quantum dot(s) may have a particle size (or anaverage particle size) of about 1 nm to about 50 nm or about 1 nm toabout 20 nm, and may be, for example, greater than or equal to about 2nm, greater than or equal to about 3 nm, or greater than or equal toabout 4 nm and less than or equal to about 50 nm, less than or equal toabout 40 nm, less than or equal to about 30 nm, less than or equal toabout 20 nm, less than or equal to about 15 nm, or less than or equal toabout 10 nm. The shapes of the quantum dot(s) are not particularlylimited. For example, the quantum dot may have a shape that includes asphere, a polyhedron, a pyramid, a multipod, a cube, a rectangularparallelepiped, a nanotube, a nanorod, a nanowire, a nanosheet, or acombination thereof, but is not limited thereto.

The quantum dot may be commercially available or may be appropriatelysynthesized. When the quantum dot is synthesized as a colloidaldispersion, the particle size of the quantum dot may be relativelyfreely and uniformly controlled.

In one embodiment, the quantum dots may not include a thiol-containingorganic compound, or a salt thereof bound to a surface of the quantumdots. In particular, the thiol-containing organic compound or the saltthereof that may not be included, or may not be present on a surface ofthe quantum dots, include butanethiol, pentanethiol, hexanethiol,heptanethiol, octanethiol, nonanethiol, decanethiol, undecanethiol,dodecanethiol, octadecanethiol, 2-(2-methoxyethoxy)ethanethiol,3-methoxybutyl 3-mercaptopropionate, 3-methoxybutylmercaptoacetate,thioglycolic acid, 3-mercaptopropionic acid, thiopronin,2-mercaptopropionic acid, a 2-mercaptopropionate ester,2-mercaptoethanol, cysteamine, 1-thioglycerol, mercaptosuccinic acid,L-cysteine, dihydrolipoic acid, 2-(dimethylamino)ethanethiol,5-mercaptomethyltetrazole, 2,3-dimercapto-1-propanol, glutathione,methoxypoly(ethylene glycol) thiol, dialkyldithiocarbamic acid or ametal salt thereof, or a combination thereof.

In an embodiment, the emission layer 13 may include a monolayercomprising a plurality of quantum dots. In another embodiment, theemission layer 13 may include at least one monolayer comprising aplurality of quantum dots, for example, 2 or more layers, 3 or morelayers, or 4 or more layers, and 20 or less layers, or 10 or lesslayers, 9 or less layers, 8 or less layers, 7 or less layers, or 6 orless layers.

The emission layer 13 may have a thickness of greater than or equal toabout 5 nm, for example, greater than or equal to about 10 nm, greaterthan or equal to about 20 nm, or greater than or equal to about 30 nmand less than or equal to about 200 nm, for example, less than or equalto about 150 nm, less than or equal to about 100 nm, less than or equalto about 90 nm, less than or equal to about 80 nm, less than or equal toabout 70 nm, less than or equal to about 60 nm, or less than or equal toabout 50 nm. The emissive layer 13 may have, for example, a thickness ofabout 10 nm to about 150 nm, for example about 10 nm to about 100 nm,for example about 10 nm to about 50 nm.

The emission layer may include a monolayer or a stacked (multi-layered)structure having at least two layers. In the multi-layered structure,adjacent layers (e.g., a first emission layer and a second emissionlayer) may have different properties and/or different composition. Theemission layer may have a halogen amount varying with a thicknessdirection thereof. In the emission layer of an embodiment, the amount ofthe halogen may increase toward the electron auxiliary layer. In theemission layer of an embodiment, the amount of the halogen may decreasetoward the electron auxiliary layer.

In an embodiment, the emission layer may have an amount of carbon thatvaries in a thickness direction thereof, For example, in an emissionlayer of an embodiment, the amount of the carbon may increase toward theelectron auxiliary layer. In the emission layer of another embodiment,the amount of the carbon may decrease toward the electron auxiliarylayer.

In an embodiment, the emission layer may include a first layer and asecond layer disposed on the first layer, and in the first layer, thequantum dot may have a halogen exchanged (e.g., chlorine-exchanged)surface and in the second layer, the quantum dot may have a halogenexchanged (e.g., chlorine-exchanged) surface. An amount (aconcentration) of an organic material (e.g., carbon) of the first layermay be less than that of the second layer. An amount (a concentration)of a halogen (e.g., chlorine) of the first layer may be greater thanthat of the second layer. An amount (a concentration) of an organicmaterial (e.g., carbon) of the first layer may be greater than that ofthe second layer. An amount (a concentration) of a halogen (e.g.,chlorine) of the first layer may be less than that of the second layer.

A HOMO energy level of the emissive layer 13 may be greater than orequal to about 5.4 eV, greater than or equal to about 5.6 eV, greaterthan or equal to about 5.7 eV, greater than or equal to about 5.8 eV,greater than or equal to about 5.9 eV, or greater than or equal to about6.0 eV. The emissive layer 13 may have a HOMO energy level of less thanor equal to about 7.0 eV, less than or equal to about 6.8 eV, less thanor equal to about 6.7 eV, less than or equal to about 6.5 eV, less thanor equal to about 6.3 eV, or than or equal to about 6.2 eV.

The emissive layer 13 may have, for example, a LUMO energy level of lessthan or equal to about 3.8 eV, less than or equal to about 3.7 eV, lessthan or equal to about 3.6 eV, less than or equal to about 3.5 eV, lessthan or equal to about 3.4 eV, less than or equal to about 3.3 eV, lessthan or equal to about 3.2 eV, or less than or equal to about 3.0 eV.The emissive layer 13 may have a LUMO energy level of greater than orequal to about 2.5 eV.

In an embodiment, the emission layer 13 may have an energy bandgap ofgreater than or equal to about 2.4 eV and less than or equal to about2.9 eV or greater than or equal to about 2.7 eV and less than or equalto about 3 eV.

In an embodiment, the light emitting device according to an embodimentmay further include a hole auxiliary layer 12. The hole auxiliary layer12 may be disposed between the first electrode 11 and the emissive layer13. The hole auxiliary layer 12 may be a single layer or a multi-layerstructure including at least two layers, wherein adjacent layers havedifferent components. As shown in FIG. 1, the hole auxiliary layer 12may include a hole injection layer (HIL), 12 a, a hole transport layer(HTL) 12 b, and/or an electron blocking layer (EBL) (not shown).

The HOMO energy level of the hole auxiliary layer 12 may be matched withthe HOMO energy level of the emissive layer 13 in order to facilitate amobility of a hole transmitted from the hole auxiliary layer 12 to theemissive layer 13. In an embodiment the hole auxiliary layer 12 may be ahole injection layer disposed near or proximate to the first electrode11 and/or a hole transport layer near to the emission layer 13.

A material included in the hole auxiliary layer 12 is not particularlylimited and may include, for example, poly(9,9-dioctyl-fluorene-co-N-(4-butylphenyl)-diphenylamine) (TFB), apoly(C6-C40)arylamine, poly(N-vinylcarbazole),poly(3,4-ethylenedioxythiophene) (PEDOT),poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS),polyaniline, polypyrrole, N,N,N′,N′-tetrakis(4-methoxyphenyl)-benzidine(TPD), 4,4-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (alpha-NPD),m-MTDATA (4,4′,4″-tris[phenyl(m-tolyl)amino]triphenylamine),4,4′,4″-tris(N-carbazolyl)-triphenylamine (TCTA),1,1-bis[(di-4-tolylamino)phenylcyclohexane] (TAPC), a p-type metal oxide(e.g., NiO, WO₃, MoO₃, etc.), a carbon-based material such as grapheneoxide, or a combination thereof, but is not limited thereto.

A thickness of the hole injection layer, the hole transport layer,and/or the electron blocking layer may be greater than or equal to about1 nm, greater than or equal to about 5 nm, greater than or equal toabout 10 nm, greater than or equal to about 15 nm, greater than or equalto about 20 nm, or greater than or equal to about 25 nm and less than orequal to about 500 nm, less than or equal to about 400 nm, less than orequal to about 300 nm, less than or equal to about 200 nm, less than orequal to about 100 nm, less than or equal to about 90 nm, less than orequal to about 80 nm, less than or equal to about 70 nm, less than orequal to about 60 nm, less than or equal to about 50 nm, less than orequal to about 40 nm, less than or equal to about 35 nm, or less than orequal to about 30 nm, but is not limited thereto.

The hole injection layer and/or hole transport layer may be formed via asolution process (e.g., spin coating). The hole injection layer and/orhole transport layer may be formed via a vapor deposition process.

In a device of an embodiment, the electron auxiliary layer 14 isdisposed between the emission layer 13 and the second electrode 12. Theelectron auxiliary layer 14 may include a first layer 14 a disposed nearto (or proximate or adjacent to) the emission layer and including afirst metal oxide; and a second layer 14 b disposed near to (orproximate or adjacent to) the second electrode and including a secondmetal oxide.

The first layer 14 a may be disposed directly on (or in contact with)the emissive layer 13 a. The second layer 14 b may be disposed directlyon (or in contact with) the first layer 14 a. The second electrode 15may be disposed directly on (or in contact with) the second layer 14 b.

In an embodiment, a device may comprise a first layer 14 a having asurface that contacts a surface of the second layer 14 b to form aninterface between the first and the second layer. An opposite surface ofthe second layer 14 b would then be in contact with a surface of thesecond electrode 15 to form an interface between the opposite surface ofthe second layer 14 b and the surface of the second electrode 15. Anopposite surface of the first layer 14 a would then be in contact with asurface of the emission layer.

In a device of an embodiment, an interface roughness between the secondlayer and the second electrode may be less than about 10 nm, less thanor equal to about 9 nm, less than or equal to about 8 nm, less than orequal to about 7 nm, less than or equal to about 6 nm, less than orequal to about 5 nm, less than or equal to about 4 nm, or less than orequal to about 3 nm.

As used herein, a determination of interface roughness (or a surfaceroughness of the interface) may be obtained based on a verticaldeviation of roughness profile (e.g., an amplitude parameter) asdetermined by a cross-sectional image of transmission or scanningelectron microscopy (e.g., Cross-TEM or Cross-SEM imaging). Theinterface roughness may also be confirmed by an atomic force microscopy.The interface roughness may be reported as an arithmetical mean or rootmean square (RMS) of the roughness profile.

The roughness profile may be obtained by using a commercial imageanalysis computer program (e.g., Image J) but is not limited thereto.

In the device, an absolute value of a difference between the workfunction of the second electrode and the conduction band edge energylevel of the second layer 14 b may be less than or equal to 0.5 eV, forexample, less than or equal to about 0.4 eV, less than or equal to about0.3 eV, less than or equal to about 0.2 eV, or less than or equal toabout 0.1 eV. In a device of an embodiment, the conduction band edgeenergy level of the first layer 14 a may be less than or equal to theconduction band edge energy level of the second layer 14 b. Theconduction band edge energy level of the first layer 14 a and theconduction band edge energy level of the second layer 14 b may bedetermined using a Kelvin probe force microscope analysis.

A quantum dot light emitting device that emits light by application of avoltage (hereinafter, QD-LED) includes quantum dots as a light emissivematerial. The QD-LED has a different light emitting principle differentfrom that of an Organic Light Emitting Diode (OLED) in terms of aluminous center. The QD-LED may realize pure colors (Red, Green, Blue),achieving improved color reproducibility, and therefore, is attractingmuch attention as a next generation display element or device. Moreover,the emission layer of the QD-LED may be prepared by a method thatinvolves a solution process, which has a commercial advantage of reducedproduction costs. Also, because the quantum dot is more of an inorganicbased material, and is not associated with a metal-ligand environment asis a phosphorescent emitter of iridium or platinum, light emission mayrealize enhanced lifetime stability over OLED.

In an embodiment, the device includes an electron auxiliary layer of theaforementioned structure having a plurality of layers of differentphysical and/or chemical properties, which may make it possible for thedevice to show enhanced electroluminescent properties and/or improvedlifetime. In order to realize a device of light emitting element withthe improved properties and lifetime demanded of a device, electrons andholes (i.e., carriers) injected from the cathode and the anode,respectively, may more readily or efficiently reach the emission layerand then recombine resulting in light emission. Referring to theschematic of FIG. 3 , exemplifying the electron flow between theelectron auxiliary layer (i.e., the first and the second layer) and thecathode (second electrode), it is believed that the electron auxiliarylayer of a two layered structure may allow an efficient electroninjection from the second electrode (pathway a) and may effectivelyblock the flow of the holes from the emission layer (pathway b), mayreduce the interface carrier recombination and the bulk non-emissiverecombination (pathway c), and may reduce the ratio of the electronsthat move away from the emission layer, i.e., in a direction against theelectron injection direction (pathway d).

The present inventors have found that in a quantum dot basedelectroluminescent device, not only is the character of the energybarrier between the second electrode (cathode) and the auxiliary layerimportant for the efficient injection of the carriers, but also, themorphology of the interface may affect the efficient injection of thecarriers. Moreover, the conduction band energy level of the auxiliarylayer (e.g., the electron auxiliary layer) and the work function of theelectrode (e.g., the metal electrode, and the interface between theauxiliary layer and the cathode may be correlated to improvement inluminous properties and lifetime of the light emitting device.

Unlike the organic based luminescent device, the QD-LED has manytechnological limitations (for example, in terms of the processtemperature and the uses of the solvent) posed on its production processafter the formation of the emission layer mainly due to the uniqueproperties of the quantum dots. In the conventional process, a solutionprocess (or a solvent treatment) may be carried out a plurality of timesduring the formation of the auxiliary layer in order to improve thecarrier injection properties. The solution process may be advantageousas it may suppress or result in a reduction of damage to the quantumdots, and at the same time the solution process may reduce theproduction cost of the device. Nevertheless, the present inventors havefound that the solution process may not be as advantageous as one mayexpect due to the formation of unspecified defects that may make itdifficult to achieve the reproducible quality required in deviceproduction. In addition, the solution process may result in irregularshape of the device element layers. Moreover, besides the difficultiesin selection of the auxiliary layer material, the underlying layer maybe substantially affected by the subsequent process in the making of theauxiliary layer, and a shape of the formed layers may be significantlyirregular, which may result in the deterioration of device propertiesand the lifetime.

In the device of the embodiment, the electron auxiliary layer mayinclude a first layer formed on the quantum dot emissive layer (e.g.,via a wet process) and a second layer having a controlled energy levelwith respect to the second electrode (or the cathode) and formed on thefirst layer. It then becomes possible to improve the electroluminescentproperties and the lifetime of the device without adversely affectingthe unique luminous properties of the quantum dots.

In the first layer, the first metal oxide may have a compositiondifferent from that of the second metal oxide of the second layer. Thefirst metal oxide may include zinc, magnesium, calcium, zirconium,yttrium, titanium, tin, tungsten, nickel, copper, cobalt, molybdenum,vanadium, gallium, manganese, iron, aluminum, niobium, cerium,strontium, barium, indium, silicon, or a combination thereof. The firstmetal oxide may include TiO₂, ZnO, SnO₂, WO₃, In₂O₃, Nb₂O₅, Fe₂O₃, CeO₂,SrTiO₃, Zn₂SnO₄, BaSnO₃, Al₂O₃, ZrO₂, SiO₂, or a combination thereof.The first metal oxide may include a zinc oxide, a zinc magnesium oxide,a tin oxide, a titanium oxide, or a combination thereof.

The first metal oxide may be represented by Chemical Formula 1:

Zn_(1-x)M_(x)O  Chemical Formula 1

In Chemical Formula 1, M is Mg, Ca, Zr, W, Li, Ti, Y, Al, or acombination thereof, and 0≤x<0.5.

The first layer may include a nanoparticle (or a plurality ofnanoparticles) of the first metal oxide. An average particle size of theplurality of nanoparticles may be greater than or equal to about 1nanometer (nm), for example, greater than or equal to 1.5 nm, greaterthan or equal to about 2 nm, greater than or equal to about 2.5 nm, orgreater than or equal to about 3 nm. An average particle size of theplurality of nanoparticles may be less than or equal to about 10nanometers, 9 nm, less than or equal to about 8 nm, less than or equalto about 7 nm, less than or equal to about 6 nm, or less than or equalto about 5 nm. It is preferred that the nanoparticles have a morespherical shape, rather than a rod shape or a nanowire shape.

In an embodiment, an amount of carbon in the first layer with respect toa molar amount of total elements in the first layer, may be greater thanor equal to about 6 mol %, greater than or equal to about 7 mol %,greater than or equal to about 8 mol %, greater than or equal to about 9mol %, greater than or equal to about 10 mol %, greater than or equal toabout 11 mol %, greater than or equal to about 12 mol %, greater than orequal to about 13 mol %, greater than or equal to about 14 mol %,greater than or equal to about 15 mol %, greater than or equal to about16 mol %, greater than or equal to about 17 mol %, greater than or equalto about 18 mol %, greater than or equal to about 19 mol %, greater thanor equal to about 20 mol %, greater than or equal to about 21 mol %,greater than or equal to about 22 mol %, greater than or equal to about23 mol %, greater than or equal to about 24 mol %, greater than or equalto about 25 mol %, greater than or equal to about 26 mol %, or greaterthan or equal to about 27 mol %.

In an embodiment, an amount of carbon in the first layer respect to amolar amount of total elements in the first layer, may be less than orequal to about 50 mol %, less than or equal to about 45 mol %, less thanor equal to about 30 mol %, less than or equal to about 29 mol %, lessthan or equal to about 28 mol %, less than or equal to about 27 mol %,less than or equal to about 26 mol %, less than or equal to about 25 mol%, less than or equal to about 24 mol %, or less than or equal to about23 mol %.

In an embodiment, an amount of carbon may be determined (or measured)(for example) by using an X-ray photoelectron spectroscopy (XPS) for anygiven layer. In an embodiment, an amount of carbon may be determined byan appropriate analysis tool such as an inductively coupled plasmaatomic emission spectroscopy, a TEM-EDX, or the like. The amount ofcarbon may be determined by preparing a cross-section sample of thedevice and carrying out an analysis on the sample.

Without wishing to be bound by any theory, the amount of carbon in theaforementioned range may contribute to protection of the emission layerduring the formation of the second layer described in detail below.

In an embodiment, the electron auxiliary layer of the device includesthe second layer that has a controlled energy level and an enhancedinterface morphology. The interface roughness between a surface of thefirst layer and a surface of the second layer may be less than or equalto about 12 nm. The surface roughness at the interface may be measuredby an analysis of the cross-section of the light emitting device (e.g.,cross-TEM or cross-SEM analysis). The interface roughness may bereported as an arithmetical mean or root mean square (RMS) of theroughness profile.

A surface roughness of the second layer and the first layer at theinterface may be less than or equal to about 11 nm, less than or equalto about 10 nm, less than or equal to about 9 nm, less than or equal toabout 8 nm, less than or equal to about 7 nm, less than or equal toabout 6 nm, less than or equal to about 5 nm, or less than or equal toabout 4 nm. A surface roughness of the second layer and the first layerat the interface may be less than or equal to about 3 nm, or less thanor equal to about 2 nm. A surface roughness of the second layer and thefirst layer at the interface may be greater than or equal to about 0.01nm, greater than or equal to about 0.1 nm, greater than or equal toabout 0.2 nm, greater than or to equal to about 0.5 nm, greater than orequal to about 0.7 nm, greater than or equal to about 0.9 nm, or greaterthan or equal to about 1 nm.

The second metal oxide of the second layer may include zinc, magnesium,calcium, zirconium, yttrium, titanium, tin, tungsten, nickel, copper,cobalt, molybdenum, vanadium, gallium, manganese, iron, aluminum,niobium, cerium, strontium, barium, indium, silicon, or a combinationthereof. The second metal oxide may include TiO₂, ZnO, SnO₂, WO₃, In₂O₃,Nb₂O₅, Fe₂O₃, CeO₂, SrTiO₃, Zn₂SnO₄, BaSnO₃, Al₂O₃, ZrO₂, SiO₂, or acombination thereof.

The second metal oxide may include a zinc oxide, a zinc magnesium oxide,a tin oxide, a titanium oxide, or a combination thereof.

The second metal oxide may be represented by Chemical Formula 2:

Zn_(1-x)M_(x)O  Chemical Formula 2

In Chemical Formula 2, M is Mg, Ca, Zr, W, Li, Ti, Y, Al, or acombination thereof, and 0≤x<0.5.

In an embodiment, at least one of the first metal oxide and the secondmetal oxide (the first metal oxide and/or the second metal oxide)comprises zinc, magnesium, calcium, zirconium, yttrium, titanium, tin,tungsten, nickel, copper, cobalt, molybdenum, vanadium, gallium,manganese, iron, aluminum, niobium, cerium, strontium, barium, indium,silicon, or a combination thereof.

In an embodiment, at least one of the first metal oxide and the secondmetal oxide comprises a zinc oxide, a zinc magnesium oxide, a tin oxide,a titanium oxide, or a combination thereof.

In an embodiment, at least one of the first metal oxide and the secondmetal oxide is represented by Chemical Formula 1 as defined above.

In an embodiment, an interface roughness of (e.g., an opposite surfaceof) the second layer and (e.g., a surface of) the second electrode maybe less than or equal to about 10 nm, less than or equal to about 9 nm,less than or equal to about 8 nm, less than or equal to about 7 nm, lessthan or equal to about 6 nm, less than or equal to about 5 nm, less thanor equal to about 4 nm, less than or equal to about 3 nm, less than orequal to about 2 nm, or less than or equal to about 1 nm. In anembodiment, an interface roughness of the second layer and the secondelectrode may be less than or equal to about 3 nm.

In an embodiment, an interface roughness of the second layer and thesecond electrode may be greater than or equal to 0 nm, for example, 0.01nm, greater than or equal to about 0.1 nm, greater than or equal toabout 0.15 nm, greater than or equal to about 0.2 nm, greater than orequal to about 0.3 nm, greater than or equal to about 0.4 nm, or greaterthan or equal to about 0.5 nm.

An amount carbon in the second layer based on a total mole amount ofelements included in the second layer may be less than or equal to about12 mol %, less than or equal to about 11 mol %, less than or equal toabout 10 mol %, less than or equal to about 9 mol %, less than or equalto about 8 mol %, less than or equal to about 7 mol %, less than orequal to about 6 mol %, less than or equal to about 5 mol %, less thanor equal to about 4 mol %, less than or equal to about 3 mol %, lessthan or equal to about 2 mol %, or less than or equal to about 1 mol %.

An amount of carbon in the second layer based on a total mole amount ofelements included in the second layer, may be greater than or equal to 0mol %. An amount of carbon in the second layer based on a total moleamount of elements included in the second layer may be greater than orequal to about 0.05 mol %, greater than or equal to about 0.1 mol %,greater than or equal to about 0.15 mol %, greater than or equal toabout 0.2 mol %, greater than or equal to about 0.25 mol %, greater thanor equal to about 0.3 mol %, greater than or equal to about 0.35 mol %,greater than or equal to about 0.4 mol %, greater than or equal to about0.45 mol %, greater than or equal to about 0.5 mol %, greater than orequal to about 0.55 mol %, greater than or equal to about 0.6 mol %,greater than or equal to about 0.65 mol %, greater than or equal toabout 0.7 mol %, greater than or equal to about 0.75 mol %, greater thanor equal to about 0.8 mol %, greater than or equal to about 0.85 mol %,greater than or equal to about 0.9 mol %, greater than or equal to about0.95 mol %, or greater than or equal to about 1 mol %.

An absolute value of a difference between the conduction band edgeenergy level of the first layer and the conduction band edge energylevel of the second layer may be greater than or equal to about 0.01 eV,greater than or equal to about 0.03 eV greater than or equal to about0.05 eV (for example, 0.07 eV, greater than or equal to about 0.09 eV,greater than or equal to about 0.1 eV, greater than or equal to about0.2 eV, greater than or equal to about 0.3 eV, or greater than or equalto about 0.4 eV). An absolute value of a difference between theconduction band edge energy level of the first layer and the conductionband edge energy level of the second layer may be less than or equal toabout 1 eV, less than or equal to about 0.9 eV, less than or equal toabout 0.8 eV, less than or equal to about 0.7 eV, less than or equal toabout 0.6 eV, less than or equal to about 0.5 eV, less than or equal toabout 0.4 eV, less than or equal to about 0.3 eV, less than or equal toabout 0.2 eV, or less than or equal to about 0.1 eV.

The conduction band edge energy level of the first layer may be greaterthan or equal to the conduction band edge energy level of the secondlayer. The conduction band edge energy level of the first layer may beless than or equal to the conduction band edge energy level of thesecond layer.

The second layer 14 b may be formed by a dry process. In an embodiment,the second layer 14 b may be formed by physical vapor deposition (PVD)(e.g., sputtering, a thermal evaporation, or the like).

Prior to the formation of the QD emission layer during the production ofthe device, the formation of the layer via such a dry process may bereadily carried out. However, after the formation of the QD emissionlayer, adopting a dry process for formation of a layer (e.g., auxiliarylayer) may not be easy and may be practically impossible using acurrently available technology as process conditions necessary for theaforementioned second layer having the aforementioned morphology andproperties may have a seriously adverse effect on the quantum dotemission layer, resulting in that a final device may not even show theluminous properties. For example, the sputtering process forming thesecond layer having the aforementioned properties (e.g., the roughness)may necessarily involve using a high energy (e.g., plasma). Exposing theemission layer, and therefore the quantum dots, to such a high energy islikely to result in irreparable damage to the quantum dots.

However, in the device of the embodiment, the first layer may play arole as a buffer layer that protects the quantum dot emission layer.Moreover, the first layer can function as an electrontransport/injection layer. Accordingly, in the embodiment, the processconditions that are necessary for the second layer to have desiredproperties and morphology can be adopted without significant damage tothe emission layer or the quantum dots.

In a device of an embodiment, the second layer may exhibit asignificantly low level of contact resistance with respect to the secondelectrode in comparison with the first layer. The contact resistance maybe measured by using a Transmission Line Measurement (TLM) method, seeexperimental herein. The second layer 14 b may have a contact resistancethat is less than or equal to about 1×10⁸ ohm, for example, less than orequal to about 1×10⁷ ohm, less than or equal to about 5×10⁶ ohm, lessthan or equal to about 3×10⁶ ohm, less than or equal to about 2.6×10⁶ohm, or less than or equal to about 2.5×10⁶ ohm.

In a device of an embodiment, the second layer may have a contactresistance that is at least about 10 times, at least about 100 times, atleast about 200 times, at least about 300 times, at least about 400times, or at least about 500 times lower than a contact resistance ofthe first layer. The second layer may exhibit an electrical conductivitythat is at least about 10 times, at least about 100 times, at leastabout 200 times, at least about 300 times, at least about 400 times, orat least about 500 times greater than an electrical conductivity of thefirst layer.

A thickness of the electron auxiliary layer, the first layer, or thesecond layer may be selected appropriate taking into consideration ofthe luminous wavelength of the quantum dots, the thickness of theemission layer, or the like.

In an embodiment, a thickness of the first layer may be greater than orequal to about 2 nm, greater than or equal to about 3 nm, greater thanor equal to about 4 nm, greater than or equal to about 5 nm, greaterthan or equal to about 7 nm, greater than or equal to about 9 nm, orgreater than or equal to about 10 nm and/or less than or equal to about50 nm, less than or equal to about 40 nm, less than or equal to about 30nm, or less than or equal to about 25 nm. A thickness of the firs layermay be greater than a thickness of the second layer. A thickness of thefirst layer may be less than a thickness of the second layer.

In an embodiment, a thickness of the second layer may be greater than orequal to about 2 nm, greater than or equal to about 3 nm, greater thanor equal to about 4 nm, greater than or equal to about 5 nm, greaterthan or equal to about 7 nm, greater than or equal to about 9 nm,greater than or equal to about 10 nm, and/or less than or equal to about70 nm, less than or equal to about 60 nm, less than or equal to about 50nm, less than or equal to about 40 nm, less than or equal to about 30nm, or less than or equal to about 25 nm.

In an embodiment, a thickness of the electron auxiliary layer may begreater than or equal to about 10 nm, greater than or equal to about 11nm, greater than or equal to about 12 nm, greater than or equal to about15 nm, greater than or equal to about 20 nm, or greater than or equal toabout 25 nm and/or less than or equal to about 200 nm, less than orequal to about 100 nm, less than or equal to about 90 nm, less than orequal to about 80 nm, less than or equal to about 70 nm, less than orequal to about 60 nm, or less than or equal to about 50 nm.

Referring to FIG. 2 , the light emitting device of an embodiment mayfurther include a buffer layer including a metal compound (e.g., a metalfluoride or an organic metal compound) (e.g., directly) on or over thesecond electrode 15 and optionally a portion of the second layer 14 b ofthe electron auxiliary layer. The metal compound (e.g. the metalfluoride or the organic metal compound) may include lithium, aluminum,or a combination thereof. The organic metal compound may include anaromatic cyclic moiety, a heteroaromatic cyclic moiety, or a combinationthereof. The organic metal compound may include a Liq, Alq3, or acombination thereof. The buffer layer may also comprise a metalfluoride, e.g., lithium fluoride.

The present inventors have also found that in a device of an embodiment,when the second layer 14 b possesses the aforementioned properties(e.g., the conductivity and the surface roughness) and the devicefurther includes an organic polymeric layer (that will be describedbelow) together with the second layer, a high temperature aging processthat is carried out in order to further enhance the device propertiesmay result in serious deterioration of some of the properties of thedevice. Without wishing to be bound by any theory, such deteriorationindicates that the polymer layer may have a substantial effect on theproperties of the second layer during the high temperature agingprocess. In an embodiment, the device may further include theaforementioned buffer layer on the second electrode (and optionally onthe second layer), whereby the deterioration may be prevented, reduced,or suppressed.

The buffer layer may have a property of a non-conductor. The bufferlayer may have an electrical conductivity of less than or equal to about3×10⁻³ Siemens per meter (S/m), or less than or equal to about 2.9×10⁻³S/m, less than or equal to about 10⁻⁴ S/m, less than or equal to about10⁻⁵ S/m, or less than or equal to about 10⁻⁶ S/m. The buffer layer mayhave a thickness of greater than or equal to about 3 nm, greater than orequal to about 4 nm, greater than or equal to about 5 nm, greater thanor equal to about 6 nm, greater than or equal to about 7 nm, greaterthan or equal to about 8 nm, or greater than or equal to about 9 nm. Thebuffer layer may have a thickness of less than or equal to about 50 nm,less than or equal to about 40 nm, less than or equal to about 30 nm, orless than or equal to about 20 nm.

Referring to FIG. 2 , the light emitting device may include an organicpolymer layer disposed on, or directly disposed on, the secondelectrode. Accordingly, the organic polymer layer may not contact with a(major) surface of the second layer as the second electrode is insteaddisposed on the (major) surface. In other embodiment, in the optionalabsence of the buffer layer (as shown) the organic polymer layer maycontact a minor (side) surface of the second layer in an embodieddevice. The organic polymer layer may include a polymerization productof a monomer combination (e.g., a monomer composition) including acompound having at least one carbon-carbon double bond (hereinafter anunsaturated compound). The monomer combination (e.g., a monomercomposition) may further include a thiol compound. In an embodiment, theorganic polymer layer may not include an unsaturated carboxylic acidcompound or a polymer thereof, benzoic acid, 3-butenoic acid, crotonicacid, butyric acid, isobutyric acid, acetic acid, or a combinationthereof. The organic polymer layer may cover (e.g., encapsulate) thefirst electrode, the second electrode, the emission layer, and/or theelectron auxiliary layer. The polymerization product may include anelectrically insulating polymer. The monomer combination (e.g., amonomer composition) may further include a (mono- or poly-) thiolcompound having at least one (e.g. at least two) thiol groups. Thepolymerization product may further include a radical polymerizationproduct between the unsaturated compound and the thiol compound.

The monomer combination (e.g., composition) including a thiol compoundmay include a multiple thiol compound and the multiple thiol compoundmay be represented by Chemical Formula A:

wherein, in Chemical Formula A, R¹ is hydrogen, a substituted orunsubstituted C1 to C30 linear or branched alkyl group, a substituted orunsubstituted C6 to C30 aryl group, a substituted or unsubstituted C7 toC30 arylalkyl group, a substituted or unsubstituted C3 to C30 heteroarylgroup, a substituted or unsubstituted C3 to C30 cycloalkyl group, asubstituted or unsubstituted C3 to C30 heterocycloalkyl group, a C1 toC10 alkoxy group, a hydroxy group, —NH₂, a substituted or unsubstitutedC1 to C30 amine group (—NRR′, wherein R and R′ are independentlyhydrogen or a C1 to C30 linear or branched alkyl group, and are notsimultaneously hydrogen), an isocyanate group, a halogen, —ROR′ (whereinR is a substituted or unsubstituted C1 to C20 alkylene group and R′ ishydrogen or a C1 to C20 linear or branched alkyl group), an acyl halidegroup (—RC(═O)X, wherein R is a substituted or unsubstituted C1 to C20alkylene group and X is a halogen), —C(═O)OR′ (wherein R′ is hydrogen ora C1 to C20 linear or branched alkyl group), —CN,

—C(═O)NRR′ or —C(═O)ONRR′ (wherein R and R′ are independently hydrogenor a C1 to C20 linear or branched alkyl group), or a combinationthereof,

L₁ is a carbon atom, a substituted or unsubstituted C1 to C30 alkylenegroup, a substituted or unsubstituted C2 to C30 alkenylene group, asubstituted or unsubstituted C3 to C30 cycloalkylene group, asubstituted or unsubstituted C6 to C30 arylene group, a substituted orunsubstituted C3 to C30 heteroarylene group (e.g., quinoline, quinolone,triazine, triazinetrione moiety, etc.), a C3 to C30 heterocycloalkylenegroup, or a substituted or unsubstituted C2 to C30 alkylene group or asubstituted or unsubstituted C3 to C30 alkenylene group where at leastone methylene (—CH₂—) is replaced by a sulfonyl group (—SO₂—), acarbonyl group ((—C(═O)), an ether group (—O—), a sulfide (—S—), asulfoxide group (—SO—), an ester group (—C(═O)O—), an amide group(—C(═O)NR—, wherein R is hydrogen or a C1 to C10 alkyl group), or acombination thereof,

Y₁ is a single bond, a substituted or unsubstituted C1 to C30 alkylenegroup, a substituted or unsubstituted C2 to C30 alkenylene group, or asubstituted or unsubstituted C2 to C30 alkylene group or a substitutedor unsubstituted C3 to C30 alkenylene group where at least one methylene(—CH₂—) is replaced by a sulfonyl group (—S(═O)₂—), a carbonyl group(—O(═O)—), an ether group (—O—), a sulfide group (—S—), a sulfoxidegroup (—S(═O)—), an ester group (—C(═O)O—), an amide group (—C(═O)NR—,wherein R is hydrogen or a C1 to C10 linear or branched alkyl group), animine group (—NR—, wherein R is hydrogen or a C1 to C10 linear orbranched alkyl group), or a combination thereof,

m is an integer of 1 or greater, for example 1 to 10,

k1 is 0 or an integer of 1 or greater, for example 1 to 10,

k2 is an integer of 1 or greater, for example 1 to 10, and

the sum of m and k2 is an integer of 3 or greater, for example 3 to 20,

provided that when Y₁ is not a single bond, m does not exceed thevalence of Y₁, and the sum of k1 and k2 does not exceed the valence ofL₁.

The thiol compound (e.g., monothiol compound or the multiple thiolcompound) may include a center moiety and at least one HS—R—* groupbound to the center moiety (wherein, R is a direct bond, a substitutedor unsubstituted C1 to C30 aliphatic hydrocarbon group, a sulfonylgroup, a carbonyl group, an ether group, a sulfide group, a sulfoxidegroup, an ester group, an amide group, or a combination thereof, and *represents a point of attachment), and the center moiety is a carbonatom, a substituted or unsubstituted C1 to C30 aliphatic hydrocarbongroup, a substituted or unsubstituted C3 to C30 alicyclic hydrocarbongroup (e.g., tricycloalkane such as tricyclodecane), a substituted orunsubstituted C6 to C30 aromatic hydrocarbon group, a substituted orunsubstituted C3 to C30 heteroarylene group, a substituted orunsubstituted C3 to C30 heterocyclic group, or a combination thereof.

In the HS—R—* group bound to the center moiety, the R may be asubstituted or unsubstituted C2 to C30 aliphatic hydrocarbon group whereat least one methylene is replaced by a sulfonyl group, a carbonylgroup, an ether group, a sulfide group, a sulfoxide group, an estergroup, an amide group, or a combination thereof.

The multiple thiol compound may be represented by Chemical Formula A-1:

wherein, in Chemical Formula A-1,

L₁′ is the same as L₁ of Chemical Formula 1, and may be for example,carbon, a substituted or unsubstituted C6 to C30 arylene group, asubstituted or unsubstituted C3 to C30 heteroarylene group, asubstituted or unsubstituted C3 to C30 cycloalkylene group, or asubstituted or unsubstituted C3 to C30 heterocycloalkylene group,

Y_(a) to Y_(d) are independently a single bond, a substituted orunsubstituted C1 to C30 alkylene group, a substituted or unsubstitutedC2 to C30 alkenylene group, or a substituted or unsubstituted C2 to C30alkylene group or a substituted or unsubstituted C3 to C30 alkenylenegroup where at least one methylene (—CH₂—) is replaced by a sulfonylgroup (—S(═O)₂—), a carbonyl group (—C(═O)—), an ether group (—O—), asulfide group (—S—), a sulfoxide group (—S(═O)—), an ester group(—C(═O)O—), an amide group (—C(═O)NR—) (wherein R is hydrogen or a C1 toC10 linear or branched alkyl group), an imine group (—NR—) (wherein R ishydrogen or a C1 to C10 linear or branched alkyl group), or acombination thereof, and

R_(a) to R_(d) are independently R¹ of Chemical Formula A or SH,provided that at least two of R_(a) to R_(d) are SH.

The center moiety, e.g., L₁ or L₁′ of Chemical Formula A or ChemicalFormula A-1, may include a triazine moiety, a triazinetrione moiety, aquinoline moiety, a quinolone moiety, a naphthalene moiety, or acombination thereof.

The multiple thiol compound of Chemical Formula A may includenonanedithiol, glycol dimercaptopropionate (e.g., ethylene glycoldimercaptopropionate), trimethylolpropane tris(3-mercaptopropionate)having the structure of Chemical Formula A-2, pentaerythritoltetrakis(3-mercaptopropionate) having the structure of Chemical FormulaA-3, pentaerythritol tetrakis(2-mercaptoacetate) having the structure ofChemical Formula A-4, tris[2-(w-mercaptopropionyloxy)alkyl] isocyanuratehaving the structure of Chemical Formula A-5, a compound having thestructure of Chemical Formula A-6, a compound having the structure ofChemical Formula A-7, a compound having the structure of ChemicalFormula A-8, or a combination thereof:

wherein, in Chemical Formula A-5, R is a substituted or unsubstituted C1to C10 alkylene (e.g., methylene, ethylene, etc);

wherein, n is an integer of 1 to 20,

wherein, n is an integer of 1 to 20,

wherein, n is an integer of 1 to 20.

The multiple thiol compound may include a dimercaptoacetate compound, atri mercaptoacetate compound, a tetramercaptoacetate compound, adimercaptopropionate compound, a trimercaptopropionate compound, atetramercaptopropionate compound, an isocyanate compound including atleast two mercaptoalkylcarbonyloxyalkyl groups, an isocyanurate compoundincluding at least two mercaptoalkylcarbonyloxyalkyl groups, or acombination thereof.

The unsaturated compound may be represented by Chemical Formula 2:

wherein, X is a C2-C30 aliphatic organic group having a carbon-carbondouble bond or a carbon-carbon triple bond, a C6-C30 aromatic organicgroup having a carbon-carbon double bond or a carbon-carbon triple bond,or a C3-C30 alicyclic organic group having a carbon-carbon double bondor a carbon-carbon triple bond,

R₂ is hydrogen, a substituted or unsubstituted C1 to C30 linear orbranched alkyl group, a substituted or unsubstituted C6 to C30 arylgroup, a substituted or unsubstituted C3 to C30 heteroaryl group, asubstituted or unsubstituted C3 to C30 cycloalkyl group, a substitutedor unsubstituted C3 to C30 heterocycloalkyl group, a C1 to C10 alkoxygroup; a hydroxy group, —NH₂, a substituted or unsubstituted C1 to C30amine group (—NRR′, wherein R and R′ are independently hydrogen or a C1to C30 linear or branched alkyl group, and are not simultaneouslyhydrogen), an isocyanate group, a halogen, —ROR′ (wherein R is asubstituted or unsubstituted C1 to C20 alkylene group and R′ is hydrogenor a C1 to C20 linear or branched alkyl group), an acyl halide group(—RC(═O)X, wherein R is a substituted or unsubstituted C1 to C20alkylene group and X is a halogen), —C(═O)OR′ (wherein R′ is hydrogen ora C1 to C20 linear or branched alkyl group), —CN, or —C(═O)ONRR′(wherein R and R′ are independently hydrogen or a C1 to C20 linear orbranched alkyl group),

L₂ is a carbon atom, a substituted or unsubstituted C1 to C30 alkylenegroup, a substituted or unsubstituted C2 to C30 alkenylene group, asubstituted or unsubstituted C3 to C30 cycloalkylene group, asubstituted or unsubstituted C6 to C30 arylene group, or a substitutedor unsubstituted C3 to C30 heteroarylene group (e.g., quinoline,quinolone, triazine, triazinetrione moiety, etc.), a C3 to C30heterocycloalkylene group, or a substituted or unsubstituted C2 to C30alkylene group or a substituted or unsubstituted C3 to C30 alkenylenegroup where at least one methylene (—CH₂—) is replaced by a sulfonylgroup (—SO₂—), a carbonyl group ((—C(═O)—), an ether group (—O—), asulfide group (—S—), a sulfoxide group (—SO—), an ester group(—C(═O)O—), an amide group

(—C(═O)NR—) (wherein R is hydrogen or a C1 to C10 alkyl group), or acombination thereof,

Y₂ is a single bond, a substituted or unsubstituted C1 to C30 alkylenegroup, a substituted or unsubstituted C2 to C30 alkenylene group, or asubstituted or unsubstituted C2 to C30 alkylene group or a substitutedor unsubstituted C3 to C30 alkenylene group where at least one methylene(—CH₂—) is replaced by a sulfonyl group (—S(═O)₂—), a carbonyl group(—C(═O)—), an ether group (—O—), a sulfide group (—S—), a sulfoxidegroup (—S(═O)—), an ester group

(—C(═O)O—), an amide group (—C(═O)NR—) (wherein R is hydrogen or a C1 toC10 linear or branched alkyl group), an imine group (—NR—) (wherein R ishydrogen or a C1 to C10 linear or branched alkyl group), or acombination thereof,

n is an integer of 1 or greater, for example, 1 to 10,

k3 is 0 or an integer of 1 or greater, for example, 1 to 10,

k4 is an integer of 1 or greater, for example, 1 to 10,

the sum of n and k4 is an integer of 3 or more, for example 3 to 20, ndoes not exceed the valence of Y₂, and the sum of k3 and k4 does notexceed the valence of L₂.

In Chemical Formula 2, X may be an acrylate group, a methacrylate group,a C2 to C30 alkenyl group, a C2 to C30 alkynyl group, a substituted orunsubstituted C3 to C30 alicyclic organic group having a carbon-carbondouble bond or a carbon-carbon triple bond in the ring, a substituted orunsubstituted C3 to C30 heterocycloalkyl group having a carbon-carbondouble bond or a carbon-carbon triple bond in the ring, a C3 to C30alicyclic organic group substituted with a C2 to C30 alkenyl group or aC2 to C30 alkynyl group, or a C3 to C30 heterocycloalkyl groupsubstituted with a C2 to C30 alkenyl group or a C2 to C30 alkynyl group.

The unsaturated compound may include a center moiety and at least twoX′—R—* groups bound to the center moiety, wherein, X is a moietyincluding a carbon-carbon unsaturated bond, for example, X as defined inChemical Formula 2, R is direct bond, a substituted or unsubstituted C1to C30 aliphatic hydrocarbon group, where at least one methylene isreplaced by sulfonyl moiety, carbonyl moiety, ether moiety, sulfidemoiety, sulfoxide moiety, ester moiety, amide moiety, or a combinationthereof, and * indicates a point of attachment to the center moiety. Thecenter moiety may include a carbon atom, a substituted or unsubstitutedC1 to C30 aliphatic hydrocarbon group, a substituted or unsubstituted C3to C30 alicyclic hydrocarbon group, a substituted or unsubstituted C6 toC30 aromatic hydrocarbon group, a substituted or unsubstituted C3 to C30heteroarylene group, a substituted or unsubstituted C3 to C30heterocyclic group, or a combination thereof.

In the X′—R—*, R may be a substituted or unsubstituted C2 to C30aliphatic hydrocarbon group where at least one methylene is replaced bysulfonyl moiety, carbonyl moiety, ether moiety, sulfide moiety,sulfoxide moiety, ester moiety, amide moiety, or a combination thereof.

In the center moiety of Chemical Formula 2, L₂ may be a triazine moiety,a triazinetrione moiety, a quinoline moiety, a quinolone moiety, anaphthalene moiety, or a combination thereof.

The substituted or unsubstituted C3 to C30 alicyclic organic grouphaving the carbon-carbon double bond or the carbon-carbon triple bond inthe ring may include a norbornene group, a maleimide group, a nadimidegroup, a tetrahydrophthalimide group, or a combination thereof.

In Chemical Formula 2, L₂ may be a group including a pyrrolidine moiety,a tetrahydrofuran moiety, a pyridine moiety, a pyrimidine moiety, apiperidine moiety, a triazine moiety, a triazinetrione moiety, atricycloalkane moiety (e.g. tricyclodecane), a tricycloalkene moiety, oran isocyanurate moiety.

The unsaturated compound may be a C4 to C100 diallyl compound, a C4 toC100 triallyl compound, a C4 to C100 diallyl ether compound, a C4 toC100 triallyl ether compound, a C4 to C100 di(meth)acrylate compound, aC4 to C100 tri(meth)acrylate compound, a divinyl ether compound, or acombination thereof.

The unsaturated compound may be a C4 to 040 diallyl compound, a C4 toC40 triallyl compound, a C4 to C40 diallyl ether compound, a C4 to C40triallyl ether compound, a C4 to C40 di(meth)acrylate compound, a C4 toC40 tri(meth)acrylate compound, a divinyl ether compound, or acombination thereof.

The unsaturated compound of Chemical Formula 2 may be a compoundrepresented by Chemical Formula 2-1, Chemical Formula 2-2, or ChemicalFormula 2-3.

In Chemical Formulae 2-1 and 2-2, Z₁ to Z₃ are independently a*-Y₂-X_(n) group, which is the same as defined for Chemical Formula 2;

wherein, in Chemical Formula 2-3,

L₂′ is carbon, a substituted or unsubstituted C1 to C30 alkylene group,a substituted or unsubstituted C2 to C30 alkenylene group, a substitutedor unsubstituted C2 to C30 alkylene group wherein at least one methylene(—CH₂—) is replaced by a sulfonyl group (—S(═O)₂—), a carbonyl group(—C(═O)—), an ether group (—O—), a sulfide group (—S—), a sulfoxidegroup (—S(═O)—), an ester group (—C(═O)O—), an amide group (—C(═O)NR—,wherein R is hydrogen or a C1 to C10 linear or branched alkyl group), animine group (—NR—, wherein R is hydrogen or a C1 to C10 linear orbranched alkyl group), a C6 to C10 cycloalkylene group, or a combinationthereof; a substituted or unsubstituted C3 to C30 alkenylene groupwherein at least one methylene (—CH₂—) is replaced by a sulfonyl group(—S(═O)₂—), a carbonyl group (—C(═O)—), an ether group (—O—), a sulfidegroup (—S—), a sulfoxide group (—S(═O)—), an ester group (—C(═O)O—), anamide group (—C(═O)NR—, wherein R is hydrogen or a C1 to C10 linear orbranched alkyl group), an imine group (—NR—, wherein R is hydrogen or aC1 to C10 linear or branched alkyl group), a C6 to C10 cycloalkylenegroup, or a combination thereof, a substituted or unsubstituted C6 toC30 arylene group, a substituted or unsubstituted C3 to C30heteroarylene group, a substituted or unsubstituted C3 to C30cycloalkylene group, or a substituted or unsubstituted C3 to C30heterocycloalkylene group,

Y_(a) to Y_(d) are independently a single bond, a substituted orunsubstituted C1 to C30 alkylene group, a substituted or unsubstitutedC2 to C30 alkenylene group, or a substituted or unsubstituted C2 to C30alkylene group or a substituted or unsubstituted C3 to C30 alkenylenegroup wherein at least one methylene (—CH₂—) is replaced by a sulfonylgroup (—S(═O)₂—), a carbonyl group (—C(═O)—), an ether group (—O—), asulfide group (—S—), a sulfoxide group (—S(═O)—), an ester group(—C(═O)O—), an amide group (—C(═O)NR—, wherein R is hydrogen or a C1 toC10 linear or branched alkyl group), an imine group (—NR—, wherein R ishydrogen or a C1 to C10 linear or branched alkyl group), or acombination thereof, and

R′_(a) to R′_(d) are independently R₂ or X as defined in ChemicalFormula 2, provided that at least two of R′_(a) to R′_(d) are X asdefined in Chemical Formula 2.

The unsaturated compound may include a compound of Chemical Formula 2-4,a compound of Chemical Formula 2-5, a compound of Chemical Formula 2-6,a compound of Chemical Formula 2-7, a compound of Chemical Formula 2-8,a compound of Chemical Formula 2-9, a compound of Chemical Formula 2-10,a compound of Chemical Formula 2-11, a compound of Chemical Formula2-12, a compound of Chemical Formula 2-13, a compound of ChemicalFormula 2-14, a compound of Chemical Formula 2-15, or a combinationthereof:

wherein, in Chemical Formula 2-7, R₁ is a C1 to C20 alkylene group, or aC2 to C20 alkylene group wherein at least one methylene (—CH₂—) isreplaced by a sulfonyl group (—S(═O)₂—), a carbonyl group (—C(═O)—), anether group (—O—), a sulfide group (—S—), a sulfoxide group (—S(═O)—),an ester group (—C(═O)O—), an amide group (—C(═O)NR—, wherein R ishydrogen or a C1 to C10 linear or branched alkyl group), an imine group(—NR—, wherein R is hydrogen or a C1 to 010 linear or branched alkylgroup), or a combination thereof, and R₂ is hydrogen or a methyl group;

wherein, in Chemical Formula 2-8, R is a C1 to C10 alkyl group;

wherein, in Chemical Formula 2-9, A is hydrogen, a C1 to C10 alkylgroup, or a hydroxy group, R₁ is a direct bond (single bond), a C1 toC20 alkylene group, or a C2 to C20 alkylene wherein at least onemethylene (—CH₂—) is replaced by a sulfonyl group (—S(═O)₂—), a carbonylgroup (—C(═O)—), an ether group (—O—), a sulfide group (—S—), asulfoxide group (—S(═O)—), an ester group (—C(═O)O—), an amide group(—C(═O)NR—, wherein R is hydrogen or a C1 to C10 linear or branchedalkyl group), an imine group (—NR—, wherein R is hydrogen or a C1 to C10linear or branched alkyl group), or a combination thereof, and R₂ ishydrogen or a methyl group;

wherein, in Chemical Formula 2-10, R₁ is a single bond, a C1 to C20alkylene, or a C1 to C20 alkylene wherein at least one methylene (—CH₂—)is replaced by a sulfonyl group (—S(═O)₂—), a carbonyl group (—C(═O)—),an ether group (—O—), a sulfide group (—S—), a sulfoxide group(—S(═O)—), an ester group (—C(═O)O—), an amide group (—C(═O)NR—, whereinR is hydrogen or a C1 to C10 linear or branched alkyl group), an iminegroup (—NR—, wherein R is hydrogen or a C1 to C10 linear or branchedalkyl group), or a combination thereof, and R₂ is hydrogen or a methylgroup;

wherein, in Chemical Formula 2-11, R is a bond, a C1 to C20 alkylene, ora C2 to C20 alkylene wherein at least one methylene (—CH₂—) is replacedby a sulfonyl group (—S(═O)₂—), a carbonyl group (—C(═O)—), an ethergroup (—O—), a sulfide group (—S—), a sulfoxide group (—S(═O)—), anester group (—C(═O)O—), an amide group (—C(═O)NR—, wherein R is hydrogenor a C1 to C10 linear or branched alkyl group), an imine group (—NR—,wherein R is hydrogen or a C1 to C10 linear or branched alkyl group), ora combination thereof, and

wherein, in Chemical Formula 2-12, R is a C1 to C20 alkylene, or a C2 toC20 alkylene wherein at least one methylene (—CH₂—) is replaced by asulfonyl group (—S(═O)₂—), a carbonyl group (—C(═O)—), an ether group(—O—), a sulfide group (—S—), a sulfoxide group (—S(═O)—), an estergroup (—C(═O)O—), an amide group (—C(═O)NR—, wherein R is hydrogen or aC1 to C10 linear or branched alkyl group), an imine group (—NR—, whereinR is hydrogen or a C1 to C10 linear or branched alkyl group), or acombination thereof,

The light emitting device may emit blue light and a T50 of the lightemitting device may be greater than or equal to about 10 hours. Thelight emitting device may have a maximum external quantum efficiency(EQE) of greater than or equal to about 12%.

In another embodiment, a method of manufacturing the aforementionedlight emitting device includes, providing a first electrode, optionallyforming a hole auxiliary layer on the first electrode, forming anemission layer on the first electrode (or optionally the hole auxiliarylayer), forming an electron auxiliary layer on the emission layer asdescribed herein; and forming a second electrode (e.g. a cathode) on theelectron auxiliary layer), wherein the formation of the electronauxiliary layer includes forming a first layer including a first metaloxide disposed on, or disposed directly on, the emission layer; andforming a second layer including a second metal oxide disposed on, ordisposed directly on, the first layer.

Details of the first electrode, the hole auxiliary layer, the secondelectrode, are as described above. The technical means of forming eachof these device structure elements is well known to those of ordinaryskill, and are selected appropriately considering the type of materialsin the forming of each structural device element, and thickness of theelectrodes and the hole auxiliary layer. The manners of forming mayinclude a solution process, a deposition process, or a combinationthereof. In an embodiment, the aforementioned hole auxiliary layer 12,the emissive layer including quantum dots 13, and the electron auxiliarylayer 14 may be formed with a solution process, for example spincoating, slit coating, inkjet printing, nozzle printing, spraying,and/or a doctor blade coating, but is not limited thereto.

The forming of the emission layer may be performed by dispersing thequantum dots in a solvent (e.g., organic solvent) to obtain a quantumdot dispersion and applying or depositing the quantum dot dispersion onthe substrate or the charge auxiliary layer in an appropriate manner(e.g., spin coating, inkjet printing, etc.). The forming of the emissionlayer may further include heat-treating the applied or deposited quantumdot layer. The heat-treating temperature is not particularly limited,and may be appropriately selected considering a boiling point of theorganic solvent. For example, the heat-treating temperature may begreater than or equal to about 60° C. The organic solvent of the quantumdot dispersion is not particularly limited and thus may be appropriatelyselected. In an embodiment, the organic solvent may include a(substituted or unsubstituted) aliphatic hydrocarbon organic solvent, a(substituted or unsubstituted) aromatic hydrocarbon organic solvent, anacetate solvent, or a combination thereof.

In the formation of the electron auxiliary layer, the first layer may beformed by a wet process. The wet process may include a sol-gel process.In an embodiment, the wet process may include a dispersion obtained bydispersing nanoparticles of metal oxide in a polar solvent, applying thedispersion on the quantum dot emission layer for example by spincoating, and drying and annealing a resulting film. The polar solventmay include a C1 to C10 alcohol solvent such as methanol, or ethanol, aC2 to C20 sulfoxide solvent such as dimethyl sulfoxide, a C2 to C20amide solvent such as dimethylformamide, or a combination thereof, butis not limited thereto.

The annealing may be carried out under vacuum at a predeterminedtemperature (e.g., greater than or equal to about 60° C., or greaterthan or equal to about 70° C. and less than or equal to about 100° C.,for example, less than or equal to about 90° C., less than or equal toabout 80° C., or less than or equal to about 75° C.), but is not limitedthereto.

The second layer may be formed on the first layer. Formation of thesecond layer may be carried out by vapor deposition. Formation of thesecond layer may be carried out by a sputtering process. The target forthe sputtering process may be selected considering a composition of thesecond metal oxide. Sputtering may be carried out under an inertatmosphere, for example, an atmosphere of argon or nitrogen gas. Thesputtering process may include a RF sputtering. The sputtering mayinclude a reactive sputtering. A gas pressure for the sputtering processmay be greater than or equal to about 1 milliTorr (mTorr) and less thanor equal to about 20 mTorr (e.g., from about 2 mTorr to about 15 mTorr,from about 3 mTorr to about 10 mTorr, or from about 4 mTorr to about 5mTorr). The sputtering power may be greater than or equal to about 100Watts (W) and less than or equal to about 1000 W, for example, fromabout 150 W to about 500 W, or from about 200 W to about 300 W.

The method of making the device may further include the formation of abuffer layer disposed on the second electrode, see FIG. 2 . Details ofthe buffer layer may be the same as set forth above. The formation ofthe buffer layer may be carried out by a physical deposition (e.g.,vapor deposition, thermal evaporation or the like) or a chemicaldeposition.

The method of making the device may further include forming an organicpolymer layer disposed on the buffer layer, see FIG. 2 . The formationof the polymer layer may include providing a monomer composition (attimes, also referred to as polymer precursor mixture) including anunsaturated compound having at least two carbon-carbon unsaturatedbonds; applying the monomer combination (or the monomer composition,hereinafter referred to as monomer composition) on the second electrodeand optionally the buffer layer to form a polymer precursor layer; andconducting polymerization (e.g., cross-linking polymerization) of themonomer composition. The monomer composition may further include a thiolcompound comprising at least one (e.g. at least two) thiol group(s).Details for the organic polymer layer are the same as set forth above.

The formation of the monomer composition is not particularly limited andmay be selected appropriately. The monomer composition may include anoligomer fora desired polymer. The monomer composition may include theaforementioned unsaturated compound, a (poly- or mono-) thiol compound,or a combination thereof. The mixing may be carried out appropriately.If present, a ratio between the thiol compound and the unsaturatedcompound is not particularly limited and may be selected appropriately.

In an embodiment, a mole ratio of the unsaturated compound with respectto the (mono- or poly-) thiol compound (e.g., a mole ratio of thecarbon-carbon double bond with respect to one mole of the thiol group)may be 0.1 or greater, 0.2 or greater, 0.3 or greater, 0.4 or greater,0.5 or greater, 0.6 or greater, 0.7 or greater, 0.8 or greater, 0.9 orgreater, 1 or greater, or 2 or greater, and/or 10 or less, for example,9 or less, 8 or less, 7 or less, 6 or less, 5 or less, 4 or less, 3 orless, 2 or less, 1.9 or less, 1.8 or less, 1.7 or less, 1.6 or less, 1.5or less, 1.4 or less, or 1.3 or less.

The monomer composition may further include additional component(s) suchas an organic solvent, a mono-thiol, a mono-unsaturated compound havingone carbon-carbon double bond, a curing inhibitor, a photoinitiator, athermal initiator, or a combination thereof.

The polymerization of the monomer composition may be carried out underan atmosphere that may not include oxygen. The polymer precursor mixturemay further include a photoinitiator and the polymerization may includea photo-polymerization. The polymerization may be carried out at atemperature of greater than or equal to about 30° C., for example,greater than or equal to about 40° C., or greater than or equal to about50° C. and less than or equal to about 100° C., for example, less thanor equal to about 90° C., or less than or equal to about 80° C.

The polymerization time may be greater than or equal to about 1 minute,for example, greater than or equal to about 5 minutes, greater than orequal to about 10 minutes, or greater than or equal to about 20 minutes,and less than or equal to about 4 hours (hrs), for example, less than orequal to about 3 hrs, less than or equal to about 2 hrs, less than orequal to about 1 hrs, or less than or equal to about 1 hrs. The methodmay further include penetration/diffusion of the monomer composition.The penetration/diffusion may be carried out at a temperature of lessthan or equal to about 30° C. or at room temperature, and thepenetration/diffusion may be conducted for example a time of greaterthan or equal to about 10 minutes, for example, greater than or equal toabout 20 minutes, greater than or equal to about 30 minutes, greaterthan or equal to about 40 minutes, greater than or equal to about 50minutes, or greater than or equal to about 1 hr and less than or equalto about 10 hrs, and less than or equal to about 9 hrs, less than orequal to about 8 hrs, less than or equal to about 7 hrs, less than orequal to about 6 hrs, less than or equal to about 5 hrs, or less than orequal to about 4 hrs.

The light emitting device of the embodiment may be applied in a displaydevice or a lighting device. The light emitting device may be applied invarious electronic devices.

Hereinafter, the embodiments are illustrated in more detail withreference to examples. However, these examples are exemplary, and thepresent disclosure is not limited thereto.

EXAMPLES Analysis Methods 1. Photoluminescence Analysis

Photoluminescence (PL) spectra of the prepared nanocrystal (quantumdots) are obtained using a Hitachi F-7000 spectrometer at an irradiationwavelength of 372 nanometers (nm).

2. Ultraviolet (UV) Spectroscopy

Hitachi U-3310 spectrometer is used to perform a UV spectroscopy andobtain UV-Visible absorption spectra.

3. TEM and TEM-EDX Analysis

-   -   (1) Transmission electron microscope photographs of nanocrystals        (quantum dots) are obtained using an UT F30 Tecnai electron        microscope.    -   (2) For a cross-section of the device as prepared, TEM-EDX        analysis are also conducted.

4. X-Ray Diffraction (XRD) Analysis

An XRD analysis is performed using a Philips XPert PRO equipment with apower of 3 kilowatts (kW).

5. Metal-Semiconductor-Metal (MSM) Analysis

A MSM analysis is carried out by using a MacScience JVL equipment.

6. Transmission Line Measurement (TLM) Analysis

A TLM analysis is carried out by using a MacScience JVL equipment. On asurface of a thin film of a semiconductor to be measured forconductivity, a plurality of metal electrodes are formed at differentpositional intervals. Then, any two electrodes are selected and avoltage is applied therebetween to measure a current. By doing this, aresistance value depending on a distance between the metal electrodesare obtained and from such data, a resistivity and a conductivity can bedetermined.

7. Electroluminescence Spectroscopy

The obtained light emitting device is evaluated for anelectro-luminescence property using a Keithley 2200 source measurementequipment and a Minolta CS2000 spectroradiometer(current-voltage-luminance measurement equipment). The current,luminance, and electroluminescence (EL) depending upon a voltage appliedto the device is measured by the current-voltage-luminance measurementequipment, and thereby an external quantum efficiency (EQE) can bedetermined.

8. Lifetime Analysis

-   -   (1) T50(hr): a time taken for a given device to exhibit a 50%        reduction of the initial luminance (100%) is measured with the        device operated at 100 nit.    -   (2) T95(hr): a time taken for a given device to exhibit 95% of        the initial luminance (100%) is measured with the device        operated at 100 nit.

Preparation of Quantum Dots Reference Example 1

(1) Selenium (Se) and tellurium (Te) are dispersed in trioctylphosphine(TOP) to obtain a 2 molar (M) Se/TOP stock solution and 0.1 M Te/TOPstock solution, respectively. 0.125 millimoles (mmol) of zinc acetate isadded along with oleic acid and hexadecylamine to a reactor includingtrioctylamine and the resulting solution is heated under vacuum at 120°C. After one hour, an atmosphere of nitrogen is added to the reactor.

Subsequently, the reactor is heated up to 300° C., the prepared Se/TOPstock solution and Te/TOP stock solution are rapidly injected into thereactor in a Te:Se mole ratio of 1:25. After 60 minutes, acetone isadded to the reaction solution, and the reaction mixture is rapidlycooled to room temperature. A precipitate obtained after centrifugationis dispersed in toluene to obtain a ZnTeSe core.

(2) 1.8 millimole (mmol) (0.336 g) of zinc acetate is added along witholeic acid to a reaction flask including trioctylamine and thenvacuum-treated at 120° C. for 10 minutes. Nitrogen is then introducedinto the reaction flask, and the temperature is increased up to 180° C.The ZnTeSe core obtained above is added to the reaction flask, Se/TOPstock solution is added and then a temperature is increased up to 280°C. Then, 1M of S/TOP stock solution is added, the temperature isincreased to 320° C., the Se/TOP stock solution and S/TOP stock solutionare then added to the reaction flask in predetermined amounts. After thereaction is complete, the reactor is cooled, ethanol is added, and themixture is centrifuged. The recovered nanocrystal is dispersed intoluene to obtain a toluene dispersion of ZnTeSe/ZnSeS core/shellquantum dots.

The used amounts of the S precursor and the Se precursor are about 0.25moles and 0.6 moles pre one mole of the zinc precursor, respectively.

Reference Example 2

The quantum dots prepared in Reference Example 1 are dispersed in 5 mLof octane at a concentration of 20 milligrams per milliliter (mg/mL) toobtain quantum dot organic dispersion. Zinc chloride is dissolved inethanol to obtain a zinc chloride solution having a concentration of 10weight percent (wt %). 0.01 milliliters (mL) of the obtained zincchloride solution is added to the prepared quantum dot organicdispersion and then, stirred at 60° C. for 30 minutes to perform asurface exchange reaction. After the reaction, ethanol is added theretoto induce a precipitation, and the quantum dots are recovered throughcentrifugation.

Synthesis of Metal Oxide Nanoparticles Reference Example 3-1: Synthesisof Zn_(x)Mg_(1-x)O Nanoparticles

Zinc acetate dihydrate and magnesium acetate tetrahydrate are added intoa reactor including dimethylsulfoxide to provide a mole ratio shown inthe following chemical formula and heated at 60° C. in an airatmosphere. Subsequently, an ethanol solution of tetramethylammoniumhydroxide pentahydrate is added into the reactor in a dropwise fashionat a speed of 3 milliliters per minute (mL/min). After stirring themixture, the obtained Zn_(x)Mg_(1-x)O nanoparticles are centrifuged anddispersed in ethanol to provide an ethanol dispersion of Zn_(x)Mg_(1-x)O(x=0.85) nanoparticles.

The obtained nanoparticles are subjected to an X-ray diffractionanalysis, to confirm a crystalline structure. The obtained nanoparticlesare analyzed by a transmission electron microscopic analysis, and theresults show that the particles have an average particle size of about 3nm.

The obtained nanoparticles are measured for their UV-Vis absorptionspectrum by using UV-Vis Spectrophotometer (UV-2600, SHIMADZU), and anenergy bandgap of the nanoparticles is obtained from the band edgetangent line of the UV-Vis absorption spectrum. The results show thatthe synthesized Zn_(x)Mg_(1-x)O nanoparticles have an energy bandgap ofabout 3.52 eV to about 3.70 eV.

Reference Example 3-2: Synthesis of ZnO

ZnO nanoparticles are prepared in accordance with the same procedure asin Reference Example 3-1, except that the magnesium acetate tetrahydrateis not used in the preparation.

The resulting nanoparticles are analyzed by X-ray diffraction analysisto confirmed a crystalline structure of the ZnO crystals. The obtainedZnO nanoparticles are analyzed with transmission electron microscopy,and the results show that the particles have an average particle size ofabout 3 nm.

Example 1

[1] Manufacture of Light Emitting Device

A device illustrated in FIG. 4A is prepared by the following process.

A glass substrate deposited with indium tin oxide (ITO) is surfacetreated with UV-ozone for 15 minutes, and then spin-coated with aPEDOT:PSS solution (H. C. Starks) and heated at 150° C. for 10 minutesunder air atmosphere and heated again at 150° C. for 10 minutes under N₂atmosphere to provide a hole injection layer (HIL) having a thickness of30 nm. Subsequently,poly[(9,9-dioctylfluorenyl-2,7-diyl-co(4,4′-(N-4-butylphenyl)diphenylamine]solution (TFB) (Sumitomo) is spin-coated on the hole injection layer andheated at 150° C. for 30 minutes to provide a hole transport layer (HTL)having a thickness of 25 nm.

The quantum dot dispersion obtained from Reference Example 1 isspin-coated on the hole transport layer as obtained above, and a zincchloride ethanol solution is dropped thereon and spin-dried, then theresulting structure is heat-treated at 80° C. for 30 minutes. Then, thequantum dot dispersion obtained from Reference Example 1 is spin-coatedthereon to provide an emissive layer having a thickness of 25 nm.

A dispersion (dispersant: ethanol, optical density: 0.5 a.u) ofZn_(x)Mg_(1-x)O (x=0.85) nanoparticles obtained from Reference Example3-1 is prepared. The prepared dispersion is spin-coated on the emissivelayer and a heat treatment at 80° C. for 30 minutes is performed toprovide a first layer for an electron auxiliary layer. The first layerhas a thickness of about 20 nm.

On the first layer, a second layer is formed to obtain an electronauxiliary layer by a sputtering process in a Ar gas (100%) and plasmagas with respect to a zinc oxide target (purity: 99% or greater) at RFpower of 300 W. The second layer has a thickness of about 10 nm.

Aluminum (Al) is vacuum-deposited on a portion of the surface of theobtained electron auxiliary layer (on the second layer) having athickness of 90 nm to provide a second electrode.

A conduction band edge energy level of the ZnO layer may be measured tobe about 4.3 eV. The work-function of the Al electrode may be measuredto be about 4.2 eV. The conduction band edge energy level of the ZnMgOlayer may be measured to be less than the conduction band edge energylevel of the ZnO layer.

[2] TEM Analysis

(1) A FIB (Focused ion beam) is used to prepare a device cross-sectionsample for the prepared device, which is then subjected to a TEManalysis. The results are shown in FIG. 5A. In the prepared device, aninterface roughness between the second (Al) electrode and the secondlayer (sputtered ZnO, Sp-ZnO) is about 3.6 nm, and a surface roughnessbetween the first layer and the second layer is about 8 nm.

(2) An FIB (Focused ion beam) is used to prepare a device cross-sectionsample for the prepared device, which is then subjected to aTransmission Electron Microscopy-Energy Dispersive X-ray (TEM-EDX)analysis. The results are shown in FIG. 5B. As shown in FIG. 5B, it isconfirmed that the ZnO layer formed by the sputtering process has arelatively high presence density of Zn.

Experimental Example 1

[1] On a Si substrate, a layer of ZnMgO particles is formed using theparticles of Reference Example 3-1 and on the layer of ZnMgO particles,a layer of ZnO particles is formed using the particles prepared inReference Example 3-2 to obtain a stacked structure of Si/ZnMgO/ZnO. Theresults are shown in FIG. 6 . From FIG. 6 , it is confirmed that whenthe second layer is formed by a wet process, a surface roughness betweenthe first layer and the second layer is greater than or equal to 20 nm.

[2] TLM and MSM Analysis

A layer of ZnMgO particles is prepared using the particles of ReferenceExample 3-1, a layer of ZnO particles is prepared using the particles ofReference Example 3-2, a ZnO thin layer is prepared by theaforementioned sputtering. For the prepared three layers, a TLM analysisand an MSM analysis is made and the results are shown in Table 1 andTable 2.

TABLE 1 ZMO ZnO ZnO formed by formed by formed by Wet process Wetprocess sputtering Contact resistance (ohm)  1.5 × 10¹¹   2 × 10⁹  2.5 ×10⁶ resistivity (ohm · cm) 1,350,000 75,000  3.5 × 10³ Conductivity(S/cm) 7.41 × 10⁻⁷ 1.33 × 10⁻⁵ 2.83 × 10⁻⁴

TABLE 2 ZMO ZnO ZnO formed by formed by formed by Wet process Wetprocess sputtering ITO/ETL/Ag: 437 2156 2500 Current density at 6 V(mA/cm²)

Considering a current density at 6 V from the MSM analysis and theconductivity from the TLM analysis, the order of the conductivity of thefilm is sputtered ZnO>solution ZnO>ZnMgO. In addition, the order of thecontact resistance with the Al electrode is sputtered ZnO<solutionZnO<ZnMgO.

Comparative Example 1-1

A device is prepared in the same manner as set forth in Example 1 exceptthat the second layer is not formed (is not present) and a thickness ofthe first layer is 20 nm.

Comparative Example 1-2

A device is prepared in the same manner as set forth in Example 1 exceptthat the first layer is not formed (is not present).

Experimental Example 2: Evaluation of Electroluminescent properties ofthe device.

For the device of Example 1, Comparative Example 1-1, and ComparativeExample 1-2, electroluminescent properties are measured and the resultsare summarized in Table 3.

TABLE 3 EQE @ Max Lum. T95 T50 10,000 nit Cd hrs hrs Comparative 2.8%2360 0.02 0.3 Example 1-1 Example 1 3.5% 4660 0.15 1.3 EQE: externalquantum efficiency Max Lum: maximum luminance

The device of Comparative Example 1-2 does not show luminous properties.

Example 2

A device is prepared in the same manner as set forth in Example 1 exceptthat on the hole transport layer (HTL) as obtained, a quantum dotdispersion is spin-coated to form an emission layer having a thicknessof about 28 nm and an organic polymer layer is formed on the cathode.For the prepared device, electroluminescent properties are measured andthe results are shown in FIG. 7 .

Comparative Example 2

A device is prepared in the same manner as set forth in Example 2 exceptthat the second layer of ZnO is not formed. For the prepared device,electroluminescent properties are measured and the results are shown inFIG. 7 .

The results of FIG. 7 confirm that the properties of the device ofExample 2 having the ZnO sputtered layer (the second layer) aresignificantly improved in comparison with Comparative Example 2, whichdoes not include the ZnO sputtered layer (i.e., the second layer).

Example 3

A device is prepared in the same manner as set forth in Example 1 exceptthat the emission layer is formed by using the halogen treated quantumdots prepared by Reference Example 2. For the prepared device,electroluminescent properties are measured and the results are shown inFIG. 8 .

Comparative Example 3

A device is prepared in the same manner as set forth in Example 3 exceptthat the second layer of ZnO is not formed. For the prepared device,electroluminescent properties are measured and the results are shown inFIG. 8 .

The results of FIG. 8 confirm that the properties of the device ofExample 3 having the ZnO sputtered layer (the second layer) aresignificantly improved in comparison with Comparative Example 3, whichdoes not include the ZnO sputtered layer.

Example 4

A device is prepared in the same manner as set forth in Example 2 exceptthat after the formation of the cathode and prior to the formation ofthe organic polymer layer, a LiF buffer layer is formed by a thermalevaporation. A schematic cross-section view of the prepared device isshown in FIG. 4B.

Experimental Example 3

The device of Example 2 and the device of Example 4 are each subjectedto an oven aging process for 10 days at a temperature of 70° C. and anambient condition.

The pictures of the device of Example 2 and the device of Example 4 areshown in FIG. 9A and FIG. 9B, respectively. The results of FIG. 9Aconfirm that in the case of the device of Example 2, the oven agingprocess may result in damages to the light emitting face. The results ofFIG. 9B indicate that the application of the buffer layer between the Alelectrode and the polymer layer can minimize such damage, and the lightemitting face of the device is relatively uniform even after the ovenaging process.

Experimental Example 4

For the device of Example 4, the oven aging is carried out for five daysat a temperature of 70° C. The electroluminescent properties aremeasured for both a non-aged and an aged device and some of the resultsare summarized in Table 4 and FIG. 10 .

TABLE 4 Max. EQE Max. Max Lum. T95 T50 EQE @ 100 nits Cd/A Cd hrs hrsAfter 5 day 4.3 3.9 2.4 16350 1.75 16.3 aging * max. EQE: maximumexternal quantum efficiency * EQE@ 100 nits: external quantum efficiencyat 100 nit * max Cd/A: Max current efficiency

The results of Table 4 and FIG. 10 confirm that the device of Example 4exhibits properties that are significantly improved by the oven agingprocess.

While this disclosure has been described in connection with what ispresently considered to be practical example embodiments, it is to beunderstood that the invention is not limited to the disclosedembodiments, but, on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

DESCRIPTION OF SYMBOLS

-   -   11: first electrode    -   12: hole auxiliary layer    -   13: emission layer    -   14: electron auxiliary layer    -   15: second electrode

1. A light emitting device, comprising: a first electrode and a secondelectrode with a surface facing the first electrode; an emission layerdisposed between the first electrode and the second electrode andcomprising a quantum dot; and an electron auxiliary layer disposedbetween the emission layer and the second electrode, wherein theelectron auxiliary layer comprises a first layer proximate to theemission layer and comprising a plurality of nanoparticles comprising afirst metal oxide, and a second layer disposed on the first layer andproximate to the second electrode, the second layer comprising a secondmetal oxide, and wherein an interface roughness between the second layerand the surface of the second electrode is less than about 10 nanometersas determined by an electron microscopy analysis, and wherein the firstmetal oxide is represented by Chemical Formula 1:Zn_(1-x)M_(x)O  Chemical Formula 1 wherein M is Mg, Ca, Zr, W, Li, Ti,Y, Al, or a combination thereof, and x is greater than 0 and less thanor equal to 0.5, and wherein an amount of carbon in the second layer isless than or equal to 12 mol % based on a total mole amount of elementsincluded in the second layer.
 2. The light emitting device of claim 1,wherein the emission layer does not comprise cadmium.
 3. The lightemitting device of claim 1, wherein an amount of carbon in the firstlayer is greater than an amount of carbon in the second layer, and anamount of the carbon in the second layer with respect to a molar amountof total elements in the second layer is less than or equal to about 8mol %.
 4. The light emitting device of claim 3, wherein the amount ofthe carbon in the first layer is greater than about 6 mole percent basedon a total mole amount of elements included in the first layer.
 5. Thelight emitting device of claim 1, wherein the first metal oxide has acomposition different from the second metal oxide.
 6. The light emittingdevice of claim 1, wherein the first metal oxide and the second metaloxide comprises zinc, and a presence density of zinc in the second layeris greater than a presence density of zinc in the first layer.
 7. Thelight emitting device of claim 1, wherein the second metal oxidecomprises a zinc oxide, a zinc magnesium oxide, a tin oxide, a titaniumoxide, or a combination thereof.
 8. The light emitting device of claim1, wherein the second metal oxide is represented by Chemical Formula 1:Zn_(1-x)M_(x)O  Chemical Formula 1 wherein M is Mg, Ca, Zr, W, Li, Ti,Y, Al, or a combination thereof, and x is greater than or equal to 0 andless than or equal to 0.5.
 9. The light emitting device of claim 1,wherein the second layer comprises a vapor deposited film of the secondmetal oxide or wherein the second layer does not include a metal oxidenanoparticle.
 10. The light emitting device of claim 9, wherein anaverage particle size of the nanoparticles of the first metal oxide isgreater than or equal to about 1 nanometer and less than or equal toabout 10 nanometers.
 11. The light emitting device of claim 1, wherein asurface of the second layer is disposed directly on a surface of thefirst layer.
 12. The light emitting device of claim 11, wherein aninterface roughness between the surface of the second layer and asurface of the first layer is less than or equal to about 12 nanometersas determined by an electron microscopy.
 13. The light emitting deviceof claim 1, wherein an interface roughness between the second layer andthe surface of the second electrode is less than or equal to about 5nanometers, and a thickness of the second layer is greater than or equalto about 1 nanometer and less than or equal to about 20 nanometers.14.-21. (canceled)
 22. The light emitting device of claim 1, wherein anabsolute value of a difference between a conduction band edge energylevel of the second layer and a work function of the second electrode isless than or equal to about 0.5 electron volts or wherein a conductionband edge energy level of the first layer is less than the conductionband edge energy level of the second layer and a difference between theconduction band edge energy level of the first layer and the conductionband edge energy level of the second layer is greater than or equal toabout 0.05 electron volts.
 23. The light emitting device of claim 1,wherein the light emitting device further comprises a buffer layerdisposed on an opposite surface of the second electrode, and the bufferlayer comprises an organic metal compound, a metal fluoride, or acombination thereof.
 24. The light emitting device of claim 23, whereina metal of the organic metal compound or the metal fluoride compriseslithium, aluminum, or a combination thereof; and an organic moiety ofthe organic metal compound comprises an aromatic cyclic moiety, aheteroaromatic cyclic moiety, or a combination thereof.
 25. The lightemitting device of claim 1, wherein the light emitting device furthercomprises an organic polymer layer disposed on an opposite surface ofthe second electrode.
 26. The light emitting device of claim 25, whereinthe organic polymer layer comprises a polymerization product of amonomer combination comprising a compound having at least onecarbon-carbon double bond, and optionally a thiol compound.
 27. Thelight emitting device of claim 1, wherein the light emitting deviceemits blue light, and a T50 of the light emitting device may be greaterthan or equal to about 10 hours.
 28. A display device comprising thelight emitting device of claim 1.